Several loss of function and splice mutations in the PAK3 gene have been identified in different families of X-linked mental retardation with certain specific clinical features, such as microcephaly, mild to moderate mental retardation, oral motor dysfunction and aggressive behavior (Allen et al., 1998; Bienvenu et al., 2000; Gedeon et al., 2003; Peippo et al., 2007; Arsenault et al., 2013). The gene discussed is PAK3; the disease is microcephaly.