In this study, we examine patterns of molecular evolution in seven microcephaly genes across placental mammals, including the first comprehensive interspecific analysis of three of these loci, STIL, CEP152 and WDR62, and the first mammal-wide analysis of two further loci, CDK5RAP2 and CENPJ. We perform tests for adaptive evolution within primates, across non-primate mammals and across the combined data to test how pervasive the signature of positive selection is. The gene discussed is CDK5RAP2; the disease is microcephaly.