Equivalogs are genes, or the proteins they encode, that are conserved in function since their last common ancestral sequence. The term equivalog was coined for use in the TIGRFAMs database. It contrasts with the older term ortholog, which makes no statement about protein function, only about evolutionary history. By definition, orthologs derive from a single locus in the last common ancestral species and have diverged from each other only through the process of the separation of species. Because lateral gene transfer is rampant, at least in prokaryotes, equivalogs may not be orthologs. Because neofunctionalization occurs even in the best of families, orthologs may not be equivalogs.