Approximately 1:250,000 live
births. True microcephaly is inherited as an autosomal recessive trait. It is
genetically heterogenous, with at least five loci mapped: MCPH1 to
chromosome 8p23; MCPH2 to locus 19q13.1-q13.2; MCPH3 at 9q34; MCPH4 at
15q15-q21; and MCPH5 to 1q31, which is a mutant of the ASPM (abnormal
spindle-like, microcephaly associated) gene.