Abnormal gene promoter methylation contributes to deregulate gene expression of hematopoietic progenitors in myelodysplastic syndromes (MDS). We analyzed the gene expression profile of myelodysplastic and normal CD34+ hematopoietic stem cells (HSCs) treated in vitro with decitabine. We identified a list of candidate tumor suppressor genes, expressed at low levels in MDS HSCs and induced by hypomethylating treatment only in MDS, but not in normal HSCs. Real-time RT-PCR confirmed reduced CD9 expression in MDS CD34+ and bone marrow mononuclear cells, compared to normal controls. CD9 was specifically up-regulated by decitabine treatment in myelodysplastic CD34+ cells.Abnormal gene promoter methylation contributes to deregulate gene expression of hematopoietic progenitors in myelodysplastic syndromes (MDS). We analyzed the gene expression profile of myelodysplastic and normal CD34+ hematopoietic stem cells (HSCs) treated in vitro with decitabine. We identified a list of candidate tumor suppressor genes, expressed at low levels in MDS HSCs and induced by hypomethylating treatment only in MDS, but not in normal HSCs. Real-time RT-PCR confirmed reduced CD9 expression in MDS CD34+ and bone marrow mononuclear cells, compared to normal controls. CD9 was specifically up-regulated by decitabine treatment in myelodysplastic CD34+ cells