PSIBLAST_PairwiseStatSig: reordering PSI-BLAST hits using pairwise statistical significance

We present an add-on to BLAST and PSI-BLAST programs to reorder their hits using pairwise statistical significance. Using position-specific substitution matrices to estimate pairwise statistical significance has been recently shown to give promising results in terms of retrieval accuracy, which motivates its use to refine PSI-BLAST results, since PSI-BLAST also constructs a position-specific substitution matrix for the query sequence during the search. The obvious advantage of the approach is more accurate estimates of statistical significance because of pairwise statistical significance, along with the advantage of BLAST/PSI-BLAST in terms of speed.