FGDY is caused by loss-of-function mutations in the FGD1 gene that encodes a 961 amino acid FGD1 protein that acts as a specific GEF for the small Rho GTPase CDC42 (Pasteris et al., 1994; Zheng et al., 1996). The gene discussed is FGD1; the disease is Aarskog-Scott syndrome, X-linked.