The BCL-2 proteins share a BCL-2 homology (BH) domain. The antiapoptotic BCL-2 proteins (BCL-2, BCL-XL, BCL-w, MCL-1, BCL2A1 and BCL-B) have four BH domains and the proapoptotic BAK, BAX and BOK proteins have three BH domains, while the proapoptotic BIM, BID, PUMA, NOXA, BAD, BMF, BIK, and HRK proteins have only BH3 domain. The location of the transmembrane (TM) domain is indicated in the figure. Of the BH3-only proteins, only BIK and HRK have a TM domain. The panels representing the three groups of proteins are not drawn to scale, and the amino acid lengths shown represent the canonical size.