BLAT Suite Program Specifications and User Guide

This documentation for the BLAT program suite explains how to use the software, including command line switches and options. Licensed users may contact us for additional tech support.


BLAT produces two major classes of alignments: at the DNA level between two sequences that are of 95% or greater identity, but which may include large inserts, and at the protein or translated DNA level between sequences that are of 80% or greater identity and may also include large inserts. The output of BLAT is flexible. By default it is a simple tab-delimited file which describes the alignment, but which does not include the sequence of the alignment itself. Optionally it can produce BLAST and WU-BLAST compatable output as well as a number of other formats.

The main programs in the BLAT suite are:

  • gfServer – a server that maintains an index of the genome in memory and uses the index to quickly find regions with high levels of sequence similarity to a query sequence.
  • gfClient – a program that queries gfServer over the network, and then does a detailed alignment of the query sequence with regions found by gfServer
  • blat –combines client and server into a single program, first building the index, then using the index, and then exiting.
  • webBlat – a web based version of gfClient that presents the alignments in an interactive fashion.

Building an index of the genome typically takes 10 or 15 minutes. Typically for interactive applications one uses gfServer to build a whole genome index. At that point gfClient or webBlat can align a single query within few seconds. If one is aligning a lot of sequences in a batch mode then blat can be more efficient, particularly if run on a cluster of computers. Each blat run is typically done against a single chromosome, but with a large number of query sequences.

Other programs in the BLAT suite are:

  • pslSort – combines and sorts the output of multiple blat runs. (The blat default output format is .psl).
  • pslReps – selects the best alignments for a particular query sequence, using a ‘near best in genome’ approach.
  • pslPretty – converts alignments from the psl format, which is tab-delimited format and does not include the bases themselves, to a more readable alignment format.
  • pslCat - Concatenates PSL files.
  • faToTwoBit – convert Fasta format sequence files to a dense randomly-accessable .2bit format that gfClient can use.
  • twoBitToFa – convert from the .2bit format back to fasta
  • faToNib – convert from Fasta to a somewhat less dense randomly accessible format that predates .2bit. Note each .nib file can only contain a single sequence.
  • nibFrag – convert portions of a nib file back to fasta.

Running the Programs

The command line options of each of the programs is described below. Similar summaries of usage are printed when a command is run with no arguments. For information on installing webBlat, see Setting up WebBlat (below).

blat - Standalone BLAT sequence search command line tool


    blat database query [-ooc=11.ooc] output.psl


    database is either a .fa file, a .nib file, or a list of .fa or .nib files.
    query is similarly a .fa, .nib, or list of .fa or .nib files.
    -ooc=11.ooc tells the program to load over-occurring 11-mers from an external file. This will increase the speed by a factor of 40 in many cases, but is not required.
    output.psl is where to put the output.


      Database type; type is one of:
      dna - DNA sequence
      prot - Protein sequence
      dnax - DNA sequence translated in six frames to protein
      The default is dna.
      Query type; type is one of:
      dna - DNA sequence
      rna - RNA sequence
      prot - Protein sequence
      dnax - DNA sequence translated in six frames to protein
      rnax - DNA sequence translated in three frames to protein
      The default is dna.
      Synonymous with -d=prot -q=prot
      Use overused tile file N.ooc. N should correspond to the tileSize.
      Sets the size of match that triggers an alignment.
      Usually between 8 and 12.
      Default is 11 for DNA and 5 for protein.
      If set to 1 this allows one mismatch in tile and still triggers an alignment. Default is 0.
      Sets the number of tile matches. Usually set from 2 to 4. Default is 2 for nucleotide, 1 for protein.
      Sets minimum score. This is twice the matches minus the mismatches minus some sort of gap penalty. Default is 30.
      Sets minimum sequence identity (in percent). Default is 90 for nucleotide searches, 25 for protein or translated protein searches.
      Sets the size of maximum gap between tiles in a clump. Usually set from 0 to 3. Default is 2. Only relevant for minMatch > 1.
      Suppress .psl header (so it's just a tab-separated file).
      Make overused tile file.
      Sets the number of repetitions of a tile allowed before it is marked as overused. Typically this is 256 for tileSize 12, 1024 for tile size 11, 4096 for tile size 10. Default is 1024. Typically only comes into play with makeOoc.
      Mask out repeats. Alignments won't be started in masked region but may extend through it in nucleotide searches. Masked areas are ignored entirely in protein or translated searches. Types are:
        lower - Mask out lower cased sequence.
        upper - Mask out upper cased sequence.
        out - Mask according to database.out RepeatMasker .out file.
        file.out - Mask database according to RepeatMasker file.out.
      Mask out repeats in query sequence. Similar to -mask above but for query rather than target sequence.
      Minimum percent divergence of repeats to allow them to be unmasked. Default is 15. Only relevant for masking using RepeatMasker .out files.
      Outputs a dot every N sequences to show program's progress.
      Trim leading poly-T.
      Don't trim trailing poly-A.
      Remove poly-A tail from qSize as well as alignments in psl output.
      Controls output file format; type is one of:
        psl - Tab separated format without actual sequence (default).
        pslx - Tab separated format with sequence.
        axt - blastz-associated axt format.
        maf - multiz-associated maf format.
        wublast - Similar to wublast format.
        blast - Similar to NCBI blast format.
      For high quality mRNAs look harder for small initial and terminal exons. Not recommended for ESTs.

Common usage scenarios

Here are some BLAT settings you may find handy for specific alignment scenarios.

1) Mapping ESTs to the genome within the same species:


2) Mapping full length mRNAs to the genome in the same species:

-ooc=11.ooc -fine -q=rna

3) Mapping ESTs to the genome across species:

-q=dnax -t=dnax

4) Mapping mRNA to the genome across species:

-q=rnax -t=dnax

5) Mapping proteins to the genome:

-q=prot -t=dnax

6) Mapping DNA to DNA in the same species:

-ooc=11.ooc -fastMap

7) Mapping DNA from one species to another species:

When mapping DNA from one species to another the query side of the alignment should be cut up into chunks of 25kb or less for best performance.

gfServer - Make a server to quickly find where DNA occurs in genome

To set up a server:

    gfServer start host port file(s).nib

To remove a server:

    gfServer stop host port

To query a server with DNA sequence:

    gfServer query host port probe.fa

To query a server with protein sequence:

    gfServer protQuery host port probe.fa

To query a server with translated dna sequence:

    gfServer transQuery host port probe.fa

To process one probe fa file against a .nib format genome (not starting server):

    gfServer direct probe.fa file(s).nib

To figure out usage level:

    gfServer status host port

To get input file list:

    gfServer files host port


      Size of n-mers to index. Default is 11 for nucleotides, 4 for proteins (or translated nucleotides).
      Number of n-mer matches that trigger detailed alignment Default is 2 for nucleotides, 3 for protiens.
      Number of insertions or deletions allowed between n-mers. Default is 2 for nucleotides, 0 for protiens.
      Translate database to protein in 6 frames. Note: it is best to run this on RepeatMasked data in this case.
      Keep a log file that records server requests.
      Include sequences in log file.

gfClient - A client for the genomic finding program


    gfClient host port nibDir in.fa out.psl


    host is the name of the machine running the gfServer.
    port is the same as you started the gfServer with.
    nibDir is the path of the nib files relative to the current dir. (Note these are needed by the client as well as the server.)
    in.fa a fasta format file. May contain multiple records.
    out.psl where to put the output.


      Database type; type is one of:
        dna - DNA sequence
        prot - Protein sequence
        dnax - DNA sequence translated in six frames to protein
      The default is dna.
      Query type; type is one of:
        dna - DNA sequence
        rna - RNA sequence
        prot - Protein sequence
        dnax - DNA sequence translated in six frames to protein
        rnax - DNA sequence translated in three frames to protein
      Outputs a dot every N query sequences.
      Suppresses psl five line header.
      Sets minimum score. This is twice the matches minus the mismatches minus some sort of gap penalty. Default is 30.
      Sets minimum sequence identity (in percent). Default is 90 for nucleotide searches, 25 for protein or translated protein searches.
      Controls output file format; type is one of:
        psl - Tab separated format without actual sequence (default).
        pslx - Tab separated format with sequence.
        axt - blastz-associated axt format.
        maf - multiz-associated maf format.
        wublast - Similar to wublast format.
        blast - Similar to NCBI blast format.
      Sets maximum intron size. Default is 750000.


webBlat generally is not run from the command line. See Setting up WebBlat (below) for information on this program.

faToTwoBit - Convert DNA from fasta to 2bit format


    faToTwoBit in.fa [in2.fa in3.fa ...] out.2bit


    -noMask - Ignore lower-case masking in fa file.

twoBitToFa - Convert all or part of .2bit file to fasta


    twoBitToFa input.2bit output.fa


    -seq=name - Restrict this to just one sequence.
    -start=X - Start at given position in sequence (zero-based).
    -end=X - End at given position in sequence (non-inclusive).

faToNib - Convert from .fa to .nib format


    faToNib in.fa out.nib

nibFrag - Extract part of a nib file as .fa


    nibFrag file.nib start end strand out.fa

pslSort - merge and sort psCluster .psl output files


    pslSort dirs[1|2] outFile tempDir inDir(s) - This will sort all of the .psl files in the directories inDirs in two stages - first into temporary files in tempDir and second into outFile. The device on tempDir needs to have enough space (typically 15-20 gigabytes if processing a whole genome).
    pslSort g2g[1|2] outFile tempDir inDir(s) -This will sort a genome to genome alignment, reflecting the alignments across the diagonal.
    Adding 1 or 2 after the dirs or g2g will limit the program to only the first or second pass repectively of the sort


    -verbose=N Set verbosity level, higher for more output. Default is 1.
    Note for huge files pslSort will run out of memory. The unix sort command:
      sort –k 10 *.psl > sorted.psl
    may be preferable in these situations, though the psl header lines should be removed or avoided with the –noHead option to blat.

pslReps - analyse repeats and generate genome-wide best alignments from a sorted set of local alignments


    pslReps in.psl out.psl out.psr


    in.psl is an alignment file generated by psLayout and sorted by pslSort
    out.psl is the best alignment output
    out.psr contains repeat info


      Don't add PSL header.
      Will not weigh in favor of larger alignments so much.
      Will not penalize for not having introns when calculating size factor.
      Takes single best hit, not splitting into parts.
      Minimum coverage to output. Default is 0.
      Ignore 'N's when calculating minCover.
      Minimum alignment ratio. Default is 0.93.
      How much can deviate from top and be taken. Default is 0.01.
      Minimum size of alignment that is near top for aligmnent to be kept. Default is 30.
      Tab-separate file with effective query sizes. When used with -minCover, this allows polyAs to be excluded from the coverage calculation.

pslPretty - Convert PSL to human readable output


    pslPretty in.psl target.lst query.lst pretty.out


    -axt - Save in Scott Schwartz's axt format.
    -dot=N Put out a dot every N records.
    -long - Don't abbreviate long inserts.

It's a really good idea if the psl file is sorted by target if it contains multiple targets. Otherwise this will be very very slow. The target and query lists can either be fasta files, nib files, or a list of fasta and/or nib files one per line. Currently this only handles nucleotide based psl files.

pslCat - Concatenate psl files


    pslCat file(s)


    -check - Parses input. Detects more errors but slower.
    -nohead - Omits psl header.
    -dir - Files are directories (Concatenate all files in dirs.)
    -out=file - Put output to file rather than stdout. - Limit files in directories to those with extension.

Setting Up webBlat


Installing a web-based BLAT Server involves four major steps:

    1) Creating sequence databases.
    2) Running the gfServer program to create in-memory indexes of the databases.
    3) Editing the webBlat.cfg file to tell it what machine and port the gfServer(s) are running on, and optionally customizing the webBlat appearance to users.
    4) Copying the webBlat executable and webBlat.cfg to a directory where the web server can execute webBlat as a CGI.


You create databases with the program faToTwoBit. Typically you'll create a separate database for each genome you are indexing. Each database can contain up to four billion bases of sequence in an unlimited number of records. The databases for webPcr and webBlat are identical.

The input to faToTwoBit is one or more fasta format files each of which can contain multiple records. If the sequence contains repeat sequences, as is the case with vertebrates and many plants, the repeat sequences can be represented in lower case and the other sequence in upper case. The gfServer program can be configured to ignore the repeat sequences. The output of Page 10 of 13 faToTwoBit is a file which is designed for fast random access and efficient storage. The output files store four bases per byte. They use a small amount of additional space to store the case of the DNA and to keep track of runs of N's in the input. Non-N ambiguity codes such as Y and U in the input sequence will be converted to N.

Here's how a typical installation might create a mouse and a human genome database:

    cd /data/genomes
    mkdir twoBit
    faToTwoBit human/hg16/*.fa twoBit/hg16.2bit
    faToTwoBit mouse/mm4/*.fa twoBit/mm4.2bit

There's no need to put all of the databases in the same directory, but it can simplify bookkeeping.

The databases can also be in the .nib format which was used with blat and gfClient/gfServer until recently. The .nib format only packed 2 bases per byte, and could only handle one record per nib file. Recent versions of blat and related programs can use .2bit files as well.


The gfServer program creates an in-memory index of a nucleotide sequence database. The index can either be for translated or untranslated searches. Translated indexes enable protein-based blat queries and use approximately two bytes per unmasked base in the database. Untranslated indexes are used nucleotide-based blat queries as well as for In-silico PCR. An index for normal blat uses approximately 1/4 byte per base. For blat on smaller (primer-sized) queries or for In-silico PCR a more thorough index that requires 1/2 byte per base is recommended. The gfServer is memory intensive but typically doesn not require a lot of CPU power. Memory permitting multiple gfServers can be run on the same machine.

A typical installation might go:

    ssh bigRamMachine
    cd /data/genomes/twoBit
    gfServer start bigRamMachine 17779 hg16.2bit &
    gfServer -trans -mask start bigRamMachine 17778 hg16.2bit &
      the -trans flag makes a translated index. It will take approximately 15 minutes to build an untranslated index, and 45 minutes to build a translate index.
      To build an untranslated index to be shared with In-silico PCR use:
        gfServer -stepSize=5 bigRamMachine 17779 hg16.2bit &
          This index will be slightly more sensitive, noticeably so for small query sequences, with blat.


The webBlat.cfg file tells the webBlat program where to look for gfServers and for sequence. The basic format of the .cfg file is line oriented with the first word of the line being a command. Blank lines and lines starting with # are ignored. The webBlat.cfg and webPcr.cfg files are similar.

The webBlat.cfg commands are:

    gfServer - Defines host and port a (untranslated) gfServer is running on, the associated sequence directory, and the name of the database to display in the webPcr web page.
    gfServerTrans - Defines location of a translated server.
    background - Defines the background image if any to display on web page.
    company - Defines company name to display on web page.
    tempDir - Where to put temporary files. This path is relative to where the web server executes CGI scripts. It is good to remove files that haven't been accessed for 24 hours from this directory periodically, via a cron job or similar mechanism.

The background and company commands are optional. The webBlat.cfg file must have at least one valid gfServer or gfServerTrans line, and a tempDir line.

Here is a webBlat.cfg file that you might find at a typical installation:

    company Awesome Research Amalgamated
    background /images/dnaPaper.jpg
    gfServer bigRamMachine 17778 /data/genomes/2bit/hg16.2bit Human Genome
    gfServer bigRamMachine 17779 /data/genomes/2bit/hg16.2bit Human Genome
    gfServer mouseServer 17780 /data/genomes/2bit/mm4.2bit Mouse Genome
    gfServer mouseServer 17781 /data/genomes/2bit/mm4.2bit Mouse Genome
    tempDir ../trash


The details of this step vary highly from web server to web server. On a typical Apache installation it might be:

    ssh webServer
    cd kent/webBlat
    cp webBlat webBlat.cfg /usr/local/apache/cgi-bin
    mkdir /usr/local/apache/trash
    chmod 777 /usr/local/apache/trash

assuming that you've put the executable and config file in kent/webBlat.

The program will create some files in the trash directory. It is good to periodically clean out old files from this directory.

On Mac OS-X instead you might do:

    cp webBlat webBlat.cfg /Library/WebServer/CGI-Executables
    mkdir /Library/WebServer/trash
    chmod 777 /Library/WebServer/trash

Unless you are administering your own computer you will likely need to ask your local system administrators for help with this part of the webBlat installation.

File Formats

.psl files

A .psl file describes a series of alignments in a dense, easily parsed text format. It begins with a five line header which describes each field. Following this is one line for each alignment with a tab between each field. The fields are described below in a format suitable for many relational databases.

matches int unsigned , # Number of bases that match that aren't repeats
misMatches int unsigned , # Number of bases that don't match
repMatches int unsigned , # Number of bases that match but are part of repeats
nCount int unsigned , # Number of 'N' bases
qNumInsert int unsigned , # Number of inserts in query
qBaseInsert int unsigned , # Number of bases inserted in query
tNumInsert int unsigned , # Number of inserts in target
tBaseInsert int unsigned , # Number of bases inserted in target
strand char(2) , # + or - for query strand, optionally followed by + or – for target strand
qName varchar(255) , # Query sequence name
qSize int unsigned , # Query sequence size
qStart int unsigned , # Alignment start position in query
qEnd int unsigned , # Alignment end position in query
tName varchar(255) , # Target sequence name
tSize int unsigned , # Target sequence size
tStart int unsigned , # Alignment start position in target
tEnd int unsigned , # Alignment end position in target
blockCount int unsigned , # Number of blocks in alignment
blockSizes longblob , # Size of each block in a comma separated list
qStarts longblob , # Start of each block in query in a comma separated list
tStarts longblob , # Start of each block in target in a comma separated list

Currently the program does not distinguish between matches and repMatches. repMatches is always zero. There is a little gotcha in the .psl format. It has to do with how coordinates are handled on the negative strand. In the qStart/qEnd fields the coordinates are where it matches from the point of view of the forward strand (even when the match is on the reverse strand). However on the qStarts[] list, the coordinates are reversed.

Here's an example of a 30-mer that has 2 blocks that align on the minus strand and 2 blocks on the plus strand (this sort of stuff happens in real life in response to assembly errors sometimes).

0 1 2 3 tens position in query
0123456789012345678901234567890 ones position in query
++++ +++++ plus strand alignment on query
-------- ---------- minus strand alignment on query

Plus strand:

qStart 12 qEnd 31 blockSizes 4,5 qStarts 12,26

Minus strand:

qStart 4 qEnd 26 blockSizes 10,8 qStarts 5,19

Essentially the minus strand blockSizes and qStarts are what you would get if you reverse complemented the query.However the qStart and qEnd are non-reversed. To get from one to the other:

qStart = qSize - revQEnd
qEnd = qSize - revQStart

.2bit files

A .2bit file can store multiple DNA sequence (up to 4 gig total) in a compact randomly accessible format. The two bit files contain masking information as well as the DNA itself. The file begins with a 16 byte header containing the following fields:

    1) signature – the number 0x1A412743 in the architecture of the machine that created the file.
    2) version – zero for now. Readers should abort if they see a version number higher than 0.
    3) sequenceCount – the number of sequences in the file
    4) reserved – always zero for now.

All fields are 32 bits unless noted. If the signature value is not as given, the reader program should byte swap the signature and see if the swapped version matches. If so all multiple-byte entities in the file will need to be byteswapped. This enables these binary files to be used unchanged on different architectures.

The header is followed by a file index. There is one entry in the index for each sequence. Each index entry contains three fields:

    1) nameSize – a byte containing the length of the name field
    2) name – this contains the sequence name itself, and is variable length depending on nameSize.
    3) offset – 32 bit offset of the sequence data relative to the start of the file

The index is followed by the sequence records. These contain 9 fields:

    1) dnaSize – number of bases of DNA in the sequence.
    2) nBlockCount – the number of blocks of N’s in the file (representing unknown sequence)
    3) nBlockStarts – a starting position for each block of N’s
    4) nBlockSizes – the size of each block of N’s
    5) maskBlockCount – the number of masked (lower case) blocks
    6) maskBlockStarts – starting position for each masked block
    7) maskBlockSizes – the size of each masked block
    8) packedDna – the dna packed to two bits per base as so: 00 – T, 01 – C, 10 – A, 11 – G. The first base is in the most significant 2 bits byte, and the last base in the least significant 2 bits, so that the sequence TCAG would be represented as 00011011. The packedDna field will be padded with 0 bits as necessary so that it takes an even multiple of 32 bit in the file, as this improves i/o performance on some machines.

.nib files

A .nib file describes a DNA sequence packing two bases into each byte. Each nib file contains only a single sequence. A nib file begins with a 32 bit signature which is 0x6BE93D3A in the architecture of the machine that created the file, and possibly a byte-swapped version of the same number on another machine. This is followed by a 32 bit number in the same format which describes the number of bases in the file. This is followed by the bases themselves packed two bases to the byte. The first base is packed in the high order 4 bits (nibble), the second base in the low order four bits. In C code:

byte = (base1<<4) + base2

The numerical values for the bases are:

0 – T, 1 – C, 2 – A, 3 – G, 4 – N (unknown)

The most significant bit in a nibble is set if the base is masked.


The gfServer program requires approximately 1 byte for every 3 bases in the genome it is indexing in DNA mode, and 1.5 bytes for each unmasked base in translated mode. The blat program requires approximately two bytes for each base in the genome in DNA mode, and three bytes for each base in translated mode. The other programs use relatively little memory.

Last revised: October 29, 2021