Fgd1 gene mutations result in faciogenital dysplasia (FGDY, Aarskog syndrome),an X-linked developmental disorder that adversely affects the formation of multiple skeletal structures (1). FGD1 maps to human chromosome Xp11.21 and shares a high degree of sequence identity with the FGD2 (6p21.2) and the FGD3 (9q22) proteins (1-4). FGD1 encodes a guanine nucleotide exchange factor that specifically activates the Rho GTPase Cdc42 (1,5). FGD2 is present in several diverse tissues during embryogenesis, suggesting a role in embryonic development (2). FGD3 stimulates fibroblasts to form filopodia, which are actin microspikes formed upon the stimulation of Cdc42 (1). All FGD family members contain equivalent signaling domains and a conserved structural organization, which strongly suggests that these signaling domains form a canonical core structure for members of the FGD family of RhoGEF proteins (2). These proteins control essential signals required during embryonic development (6,7).