Rac deletion impairs osteoblast growth and differentiation in vitro. Primary osteoblast cultures were transduced with an inducible Cre-expressing retrovirus (MSCV-CreER-puro) and treated with 4OHT or with EtOH as a control. (A) CFU-Fs were significantly reduced in Rac-deleted osteoblast cultures after induction of Cre expression by 4OHT (CFU-F Rac^del 19 ± 4.4 EtOH vs 7.0 ± 1.6 4OHT), but not in WT controls. (B) CFU-Alk was reduced in Rac-deleted osteoblast cultures after 4OHT treatment (CFU-Alk Rac^del 8.5 ± 1.7 EtOH vs 0.8 ± 0.5 4OHT). A modest reduction in CFU-Alk was observed with 4OHT treatment in WT osteoblast controls. (C) CFU-Alz was reduced in Rac-deleted osteoblast cultures after 4OHT treatment (CFU-Alz Rac^del 8.5 ± 1.9 EtOH vs 2.5 ± 1.3 4OHT). (D) Apoptosis (annexin V⁺ cells by flow cytometry) was increased in WT and Rac-deficient osteoblast cultures after 4OHT treatment (Rac^del 7.1% ± 2.7% EtOH vs 17.4% ± 6.1% 4OHT; WT 7.4% ± 0.5% EtOH vs 14.5% ± 2.0% 4OHT). All values are means ± SD. n = 3 per condition for each experiment.