The transcriptional repressing Polycomb-group (PcG) family is a part of a cellular memory system responsible for the stable inheritance of gene activity. PcG proteins assemble into multimeric protein complexes, which are involved in maintaining the transcriptional repressive state of genes over successive cell generations. MBTD1 (malignant brain tumor domain containing 1) is a 628 amino acid nuclear protein that belongs to the PcG family of proteins. Existing as a monomer, MBTD1 undergoes alternative splicing events to produce three isoforms. MBTD1 contains one FCS-type zinc finger and four MBT repeats, and is encoded by a gene that maps to human chromosome 17q21.33. Chromosome 17 comprises over 2.5% of the human genome and encodes over 1,200 genes. Two key tumor suppressor genes are associated with chromosome 17, namely, p53 and BRCA1.