Genomic regions and features that share a common ancestry. Although there are different definitions of this term, for comparative genomics work, this definition is the simplest. If two genes are said to be homologous with respect to one another, then they share a common ancestral gene. In such a case, these genes are homologs. N.B. Homology is not a synonym for similarity. With this definition, phrases such as “95% homologous” are nonsensical, as genomic regions or features are either homologous, or not. The only exception would be gene fusion events, whereby a new gene is created by the fusion of two other genes (or domains thereof).