What is BLAST?

BLAST (Basic Local Alignment Search Tool) is a set of similarity search programs designed to explore available sequence databases regardless of whether the query sequence is protein or DNA.  BLAST is useful because it allows scientists to compare new sequences with databases containing previously characterized genes.  Upon interpretation, the results can provide valuable functional and evolutionary information.  The scores assigned in a BLAST search have a statistical interpretation (E-value), making real matches easier to distinguish from random background hits. BLAST uses a heuristic (approximate) algorithm which seeks local as opposed to global alignments and is therefore able to detect relationships among sequences which share only isolated regions of similarity (Altschul et al., 1990).

How to access BLAST?

BLAST services are available over the web (http://www.ncbi.nlm.nih.gov/BLAST/), using a standalone BLAST, a network client, or by sending an email with your nucleotide or protein query sequence.  Standalone BLAST is useful for searching your own custom made databases.  It can be downloaded from NCBI's ftp site at ftp://ftp.ncbi.nlm.nih.gov/blast/server/current_release/.  

Other Blast tutorials can be found at:

    Introduction to Software for the Analysis of Sequence Similarity
    NCBI BLAST tutorial