Input
Blast DB	Identifies the database to search. Database must already be formatted by formatdb.
Nucleotide Query sequence(s)	If the input file contains multiple sequences, BLAST will be run on each sequence in order, and the resulting output will contain concatenated BLAST reports.
Believe the query defline.	Believe the query definition line.
Output
Result	Designates an output file for the search results.
Format	Pairwise (Default) Query-anchored, showing identities Query-anchored, no identities Flat query-anchored, showing identities Flat query-anchored, no identities Query-anchored, no identities and blunt ends Flat query-anchored, no identities and blunt ends XML Blast output Tabular Tabular with comment lines ASN, text ASN, binary
Show GI's in deflines	Shows GenInfo Identifier (GI) numbers in definition lines. A GI is a unique numeric identifier assigned for a sequence in GenBank. A GI corresponds to an accession version pair.
Produce HTML output	Produces HTML output with [anchor] links from the summary at the top of the report to the alignments farther below. This option should be used only with the standard report format ("Pairwise (Default)").
Number of Alignments to output	Truncates the report to set number of alignments. There is no warning when you exceed this limit, so it's generally a good idea to set this value very high unless you're interested only in the top hits.
SeqAlign file (Optional)	SeqAlign output file
Options
Expectation value	Sets the threshold expectation value for keeping alignments. This is the E from the Karlin-Altschul equation that describes how often an alignment with a given score is expected to occur at random.
Filter query sequence	Filters the query sequence for low-complexity subsequences. The default setting is ON. Complexity filtering is generally a good idea, but it may break long HSPs into several smaller HSPs due to low-complexity segments. This can cause some alignments to fall below the significance threshold and be lost. To prevent this, either turn off filtering (not recommended) or use soft masking, in which the filter is used only in the word seeding phase, but not the extension phase. DUST with blastn, SEG with others.
Gapped Alignment X dropoff value	X dropoff value for gapped alignment (in bits); Zero invokes default behavior; blastn 30, megablast 20, tblastx 0, all others 15.
Number of DB Seqs to show descriptions	Sets the number of database sequences for which to show the one-line summary descriptions at the top of a BLAST report. You won't be warned if you exceed a value. Also see the "Number of Alignments to output" parameter.
Extending Hits Threshold	Neighborhood word threshold score. Only those words scoring equal to or greater than [value] will seed alignments. Zero is default; blastp 11, blastn 0, blastx 12, tblastn 13, tblastx 13, megablast 0.
Translation table	Select translation table.
DB Genetic code	The genetic code to use for translation of the database nucleotide sequence. See http://www.ncbi.nlm.nih.gov/htbin-post/Taxonomy for updates
Matrix	Designates a protein similarity matrix. This is used in all BLAST programs except blastn. Matrices are sought in the following order: in the local directory, in the location specified in the .ncbirc file, in a local data directory, and finally, in the BLASTMAT environment variable (only on Unix systems). Other matrices included in the standard distribution include BLOSUM45, BLOSUM80, PAM30, and PAM70. You can use custom matrix files, but it requires modifying the source code and defining the new matrix with all of its associated statistics for different affine gap combinations and recompiling the binary. Using these custom files isn't recommended because it requires the arduous task of calculating gapped values for lambda and maintaining a derivative branch of the source code.
Word size	Sets the word size for the initial word search. Word sizes for blastp, blastx, tblastn, and tblastx are 2 or 3.
DataBase Effective Length	Effective length of the database. Use zero for the real size (Default).
Best Hits Number	The number of best hits from a region to keep. This option is useful when you want to limit the number of alignments that might pile up in one section of the query. This is most useful if the settings of "Number of Alignments to output" or "Number of DB Seqs to show descriptions" are low, and the abundant alignments push lower scoring alignments off the end of the report. Off by default, if used a value of 100 is recommended.
Two-hit or Single-hit Algorithm	Specifies the two-hit or single-hit algorithm. The two-hit option requires two word hits on the same diagonal to extend from either one. When set to two-hit mode, the "Multiple Hits Window Size" parameter specifies how close the two hits have to be to trigger extension.
Query strands	Chooses which strand of DNA-based queries is searched. Top Strand Bottom Strand Both Strands
Location on query sequence	The location on query sequence. This lets you limit the search to a subsequence of the query sequence. For example, to search just the letters from 21 to 50, set the parameter to following: "21,50". The alignments won't extend outside the specified region. In older versions of BLAST, this parameter set the size of the region under control of the "Best Hits Number" parameter.
Search Space Effective Length	Effective length of the search space. Use zero for the real size (Default).
Lower Case Filtering	Use lower case filtering of FASTA sequence.
Ungapped Extension X dropoff value	X dropoff value for ungapped extensions in bits; Zero invokes default behavior; blastn 20, megablast 10, all others 7.
Final Gapped Alignment X dropoff value	X dropoff value for final gapped alignment in bits; Sets the X3 dropoff value (in bits) for extensions but is bounded by the value for X2. Zero invokes default behavior; blastn/megablast 50, tblastx 0, all others 25.
Multiple Hits Window Size	Sets the multiple-hit window size [integer]. When BLAST is set to two-hit mode, this option requires two word hits on the same diagonal to be within [value] letters of each other in order to extend from either one. The larger the [value], the more sensitive BLAST will be. Setting [value] to 0 sets the default behavior of 40, except for blastn, whose default is single word hit. To specify one-hit behavior, set 1. Blastn/megablast 0 (Default), all others 40.
Number of processors	Sets the number of processors to use. If you have multiple queries, you will get better throughput by executing multiple BLAST searches. For insensitive searches such as default BLASTN, setting -a to a higher value may not appreciably improve speed if disk I/O is the bottleneck.
Old Engine Use	Force use of old engine.