The orthology relation in the process of yeast-to-human inference method. The one-to-one case of one yeast gene having one human homolog (A). The n-to-one case of several human genes having the same yeast homolog (B). The one-to-m case can be defined analogously. The n-to-m is a very rare case where more than one human gene are homologs of more than one yeast gene (C).