... lecture).¹

blastn and blastp offer fully gapped alignments. blastx and tblastn have 'in-frame' gapped alignments and use sum statistics to link alignments from different frames. tblastx provides only ungapped alignments.

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... homologues²

The PSI-BLAST program uses the information from any significant alignments returned to construct a position-specific score matrix, which replaces the query sequence for the next round of database searching. PSI-BLAST may be iterated until no new significant alignments are found.

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