Stimulation of erythroid progenitor growth by calcitriol is dependent on expression of Vdr. Lin^neg cKit⁺CD71^lo/neg cells were cultured for 2 days under progenitor conditions (PM) with shRNA lentiviruses targeting Vdr (shRNA1 and shRNA2) or luciferase. Mock treated and untreated cells were included as additional controls. (A) Proliferation of cKit⁺CD71^lo/neg cells cultured in MM with or without calcitriol (n = 3). (B) Analysis of Vdr expression in Lin^neg cKit⁺CD71^lo/neg cells after 2 days of culture in PM. Vdr expression levels were normalized to Ubb and are presented as percentage expression compared with the luciferase shRNA control (dotted line, 100%). (C-D) Frequency of enucleated (C) and Ter119⁺ (D) cells after culture for 7 days in MM. Data were analyzed using an unpaired Student t test (B-D) or using a 2-way ANOVA (A; *P < .05, A-B; **P < .01, A; ***P < .001, A). Error bars, ± SEM (n = 3).