Rac1 and Cdc42, Rho family low molecular weight G proteins, are intracellular signaling factors that transmit various information from outside to inside cells. Primarily, they are known to control various biological activities mediated by actin cytoskeleton reorganization, such as cell proliferation, differentiation, and apoptosis. In order to investigate the functions of Rac1 and Cdc42 in bone formation, we prepared cartilage-specific double conditional knockout mice, Rac1(fl/fl); Cdc42(fl/fl); Col2-Cre (Rac1: Cdc42 dcKO mice), which died just after birth, similar to Cdc42(fl/fl); Col2-Cre mice (Cdc42 cKO mice). Our findings showed that the long tubule bone in Rac1: Cdc42 dcKO mice was shorter than that in Rac1(fl/fl);Col2-Cre mice (Rac1 cKO mice) and Cdc42 cKO mice. Abnormal skeleton formation was also observed and disordered columnar formation in the growth plate of the Rac1: Cdc42 dcKO mice was more severe as compared to the Rac1 cKO and Cdc42 cKO mice. Together, these results suggest that Rac1 and Cdc42 have cooperating roles in regulation of bone development. (C) 2018 Elsevier Inc. All rights reserved.