Mammalian Polycomb group (Pc-G) genes, constituting some 5 subfamilies based on their identity to the Drosophila genesPc, Psc, ph, esc, and E(z), appear to play critical roles in maintaining the transcriptional repression state ofHox/HOM-C genes during development. Despite increasing evidence of the important role of Hox genes in both normal hematopoiesis and leukemic transformation, little is known about the expression and possible function played by Pc-G genes in hematopoietic cells. To address this, we first examined the expression of Pc genes in purified CD34⁺ human bone marrow cells by reverse transcriptase-polymerase chain reaction (RT-PCR), using degenerate primers that specifically amplify the majority of Pcgenes. This analysis showed the expression of 8 different Pcgenes in CD34⁺ bone marrow cells, includingHP1^Hsα, HP1^Hsγ, the heterochromatin p25 protein, the human homologue of the murine M32 gene, and 4 novel members of this family. To assess whether Pc-G mRNA levels change during differentiation of bone marrow cells, a quantitative RT-PCR method was used to amplify the total cDNA originating from three purified subpopulations of CD34⁺bone marrow cells known to differ in their ability to grow in long-term or semisolid cultures. In sharp contrast to Hox gene expression, which is highest in the most primitive bone marrow cells, these studies show that the expression level of 8 of the 9 Pc-Ggenes studied (ie, HP1^Hsα, HP1^Hsγ, M31, M32, M33, Mel-18, Mph1/Rae-28, and ENX-1) markedly increases with differentiation of bone marrow cells. Interestingly,BMI-1 exhibits a strikingly different pattern of expression, with high expression levels in primitive cells and very little expression in mature CD34⁻ cells. Together, these results document for the first time that differentiation of human bone marrow cells is accompanied by profound changes in Pc-G gene expression levels.