Down-regulation of p38α by siRNA can stimulate hematopoiesis in MDS CD34⁺ progenitors. A mixture of 4 siRNAs against p38α were transfected in primary CD34⁺ hematopoietic progenitors using Mirus TKO transfection reagent. Western blotting showed a specific and significant decrease in total p38 protein levels (A). High transfection efficiency was shown by using fluorescent-labeled siRNAs (B). MDS CD34⁺ cells were transfected with either anti-p38α or control-scrambled siRNAs and grown in vitro in methylcellulose with cytokines. Colonies were scored on day 14, and results were expressed as means ± SEMs of 3 independent experiments. Significantly higher number of both myeloid (CFU-GM) and erythroid (BFU-E) colonies were observed in cells transfected with anti-p38 siRNAs (C). MDS CD34⁺ cells transfected with anti-p38α and scrambled control siRNAs were evaluated after 48 hours by Annexin V staining. Flow cytometry revealed a significantly higher percentage of viable cells (P = .045, t test) and fewer number of apoptotic cells (P = .04, t test) when transfected with anti-p38α siRNA (D). Results are presented as means ± SEMs of 5 independent experiments.