Mammalian Sin 3 (mSin 3) is a protein closely related to the yeast SIN3 repressor protein involved in the transcriptional repression of many genes. There are two mammalian isoforms, mSin 3A and mSin 3B. Containing 4 paired amphipathic helix domains (PAH domains), mSin 3A and mSin 3B have been shown to directly interact with several other transcriptional repressor proteins including HDAC 1, HDAC 2, RbAp 46, the methyl CpG binding protein MeCP 2, the Mad/Max heterodimer, and the corepressors silencing mediator of retinoic acid & thyroid hormone receptor (SMRT) and nuclear receptor corepressor (N-CoR).