The reciprocal smallest distance algorithm (RSD). Arrows denote bidirectional BLAST runs. After each run, hits are paired with the query to calculate evolutionary distances. If the same pair produces the smallest distance in both search directions, it is assumed to be orthologous. The specifics of the algorithm are provided in the Introduction.
Wall et al. BMC Bioinformatics 2010 11:259 doi:10.1186/1471-2105-11-259