['Mel-18 is a member of the mammalian Polycomb group (PcG) genes. This family of genes regulates global gene expression in many biologic processes, including hematopoiesis and anterior-posterior axis formation by manipulating specific target genes, including members of the Hox family. Here, we demonstrate that mel-18 negatively regulates the self-renewal activity of hematopoietic stem cells (HSCs).', 'Long-term reconstitution activity was evaluated by competitive repopulating unit (CRU) and mean activity of the stem cells (MAS) assays in vivo in bone marrow cells (BMCs) derived from mel-18(-/-) and mel-18 tg mice. The expression levels of mel-18 and Hoxb4 were measured by quantitative real-time reverse transcription polymerase chain reaction.', 'The Hoxb4 gene was highly expressed in HSCs derived from mel-18(-/-) mice. The observed CRUs were 3.21, 4.77, 3.32, and 1.64 CRU per 10(5) BMCs in mel-18(+/+), mel-18(-/-), C57BL/6, and mel-18 tg, respectively. MAS was 0.58, 0.18, 0.41, and 5.89 in mel-18(+/+), mel-18(-/-), C57BL/6, and mel-18 tg, respectively. The percentage in G0 phase HSCs (lin(-)flk2(-)c-Kit(+)Sca1+ cells) was increased in mel-18(-/-) mice and decreased in mel-18 tg mice.'] Does polycomb group gene mel-18 modulate the self-renewal activity and cell cycle status of hematopoietic stem cells?