FGD4 (FYVE, RhoGEF and PH domain-containing 4), also known as Frabin (FGD1-related F-actin binding protein), like FGD2 and FGD3, is a Cdc42-specific GEF that shows significant sequence homology to FGD1, which was originally discovered by positional cloning as the gene responsible for a human X-linked skeletal disease called faciogenital dysplasia [30]–[33]. Here, FGD1 is linked to Aarskog-Scott syndrome, X-linked.