Primary microcephaly is a genetic disorder in which an affected individual has a head circumference >3 SDs below the age- and sex-related mean. A small but apparently normally formed brain is the reason for the reduced head circumference, and, probably because of this, all affected individuals are mentally retarded. The condition is genetically heterogeneous, and four loci have already been identified. We now report a fifth locus, MCPH5, which is an 8-cM region mapping to chromosome 1q31, defined by the markers GATA135F02 and D1S1678.