The Basic Local Alignment Search Tool (BLAST) finds regions of local similarity between sequences. The program compares nucleotide or protein sequences to sequence databases and calculates the statistical significance of matches.
BLAST can be used to infer functional and evolutionary relationships between sequences as well as help identify members of gene families.
BLAST is one of the most widely used bioinformatics programs for sequence searching.
BLAST is a registered trademark of the National Library of Medicine.
There are many different flavors of BLAST searches:
- The megaBLAST nucleotide-nucleotide search, optimized for very similar sequences (in the same or in closely related species), first looks for an exact match of 28 bases, and then attempts to extend that initial match into a full alignment.
- The BLASTN nucleotide-nucleotide search looks for more distant sequences.
- BLASTP performs protein-protein sequence comparison, and its algorithm is the basis of many other types of BLAST searches such as BLASTX and TBLASTN.
- BLASTX searches a nucleotide query against a protein database, translating the query on the fly.
- TBLASTN searches a protein query against a nucleotide database, translating the database on the fly.
- PSI-BLAST first performs a BLASTP search to collect information that it then uses to produce a Position-Specific-Scoring-Matrix (PSSM). A PSSM for a query of length N is an N x 20 matrix. Each of the N columns corresponds to a letter in the query, and each column contains 20 rows. Each row corresponds to a specific residue and describes the probability of related sequences having that residue at that position. PSI-BLAST can then search a database of protein sequences with this PSSM.
- RPSBLAST (Reverse-Position-Specific BLAST) can very quickly search a protein query against a database of PSSMs that were usually produced by PSI-BLAST.
- DELTA-BLAST produces a PSSM with a fast RPSBLAST search of the query, followed by searching this PSSM against a database of protein sequences.
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