  It is common to use reciprocal best hits, also known as a boomerang
criterion, for determining orthology between sequences. The best hits may be
found by blast, or by other more recently developed algorithms. Previous work
seems to have assumed that reciprocal best hits is a sufficient but not
necessary condition for orthology. In this article, I explain why reciprocal
best hits cannot logically be a sufficient condition for orthology. If
reciprocal best hits is neither sufficient nor necessary for orthology, it
would seem worthwhile to examine further the logical foundations of some
unsupervised algorithms that are used to identify orthologs.
