Intrinsic activity of SK-1 regulates EC differentiation. WT BM cells containing the empty vector (EV) control, WT SK-1 (SK-1), catalytically inactive mutant (G82D), or the phosphorylation-dead mutant (S225A). ECU numbers determined at day 4 of culture with results showing the mean plus or minus SEM from 3 separate experiments. SK-1 activity was determined at completion of the experiments to confirm the successful manipulation (shown numerically as U/mg protein normalized to WT^EV). *P < .05, compared with EV and G82D.