The coding region determinant-binding protein/insulin-like growth factor II mRNA-binding protein (CRD-BP/IMP1) is an RNA-binding protein specifically recognizing c-myc, leader 3' IGF-II and tau mRNAs, and the H19 RNA. CRD-BP/IMP1 is predominantly expressed in embryonal tissues but is de novo activated and/or overexpressed in various human neoplasias. To address the question of whether CRD-BP/IMP1 expression characterizes certain cell types displaying distinct proliferation and/or differentiation properties (i.e. stem cells), we isolated cell subpopulations from human bone marrow, mobilized peripheral blood, and cord blood, all sources known to contain stem cells, and monitored for its expression. CRD-BP/IMP1 was detected only in cord blood-derived CD34(+) stem cells and not in any other cell type of either adult or cord blood origin. Adult BM CD34(+) cells cultured in the presence of 5'-azacytidine expressed de novo CRD-BP/IMP1, suggesting that epigenetic modifications may be responsible for its silencing in adult non-expressing cells. Furthermore, by applying the short interfering RNA methodology in MCF-7 cells, we observed, subsequent to knocking down CRD-BP/IMP1, decreased c-myc expression, increased IGF-II mRNA levels, and reduced cell proliferation rates. These data 1) suggest a normal role for CRD-BP/IMP1 in pluripotent stem cells with high renewal capacity, like the CB CD34(+) cells, 2) indicate that altered methylation may directly or indirectly affect its expression in adult cells, 3) imply that its de novo activation in cancer cells may affect the expression of c-Myc and insulin-like growth factor II, and 4) indicate that the inhibition of CRD-BP/IMP1 expression might affect cancer cell proliferation.