Microcephalin (MCPH1) is a gene that is expressed during fetal brain development. Certain mutations in MCPH1, when homozygous, cause primary microcephaly — a severely diminished brain. Hence it has been assumed that variants have a role in brain development, but in normal individuals no effect on mental ability or behavior has yet been demonstrated in either this or another similarly studied microcephaly gene, ASPM. However, an association has been established between normal variation in brain structure as measured with MRI (i.e., primarily cortical surface area and total brain volume) and common genetic variants within both the MCPH1 gene and another similarly studied microcephaly gene, CDK5RAP2.