Polycomb group (PcG) proteins are important for maintaining the transcriptionally repressed state of target genes and are thought to function via chromatin modification. PcG proteins assemble into multimeric protein complexes, which are involved in maintaining the transcriptional repressive state of genes over successive cell generations. PcG proteins are also required for normal maturation of myeloid progenitor cells. A putative PcG protein, L3MBTL3 (lethal(3)malignant brain tumor-like protein 3), also known as MBT-1 or KIAA1798, is a 780 amino acid protein containing three MBT repeats and one SAM (sterile alpha motif) domain. Localized to the nucleus, L3MBTL3 interacts with RING1B, another PcG protein that may be involved in the specification of anterior-posterior axis and cell proliferation in early development. L3MBTL3 exists as two isoforms produced by alternative splicing events.