BLAST

BLAST stands for Basic Alignment Search Tool. Because of its speed and the reliability, BLAST is a very widely used program for sequence alignment. The statistical significance of a BLAST alignment is given by its expectation value or E-value. The E-value of an alignment with score S is defined to beThe number of different alignents with scores equivalent to or better than S that are expected to occur in a database search by chance.(Quoted from the BLAST glossary at NCBI ).