The KRAB domain is a transcription repression module, found in a subgroup of the zinc finger proteins (ZFPs) of the C2H2 family, KRAB-ZFPs. KRAB-ZFPs comprise the largest group of transcriptional regulators in mammals, and are only found in tetrapods. These proteins have been shown to play important roles in cell differentiation and organ development, and in regulating viral replication and transcription. A KRAB domain may consist of an A-box, or of an A-box plus either a B-box, a divergent B-box (b), or a C-box. Only the A-box is included in this model. The A-box is needed for repression, the B- and C- boxes are not. KRAB-ZFPs have one or two KRAB domains at their amino-terminal end, and multiple C2H2 zinc finger motifs at their C-termini. Some KRAB-ZFPs also contain a SCAN domain which mediates homo- and hetero-oligomerization. The KRAB domain is a protein-protein interaction module which represses transcription through recruiting corepressors. A key mechanism appears to be the following: KRAB-AFPs tethered to DNA recruit, via their KRAB domain, the repressor KAP1 (KRAB-associated protein-1, also known as transcription intermediary factor 1 beta , KRAB-A interacting protein , and tripartite motif protein 28). The KAP1/ KRAB-AFP complex in turn recruits the heterochromatin protein 1 (HP1) family, and other chromatin modulating proteins, leading to transcriptional repression through heterochromatin formation.