MCL1 and cancer: Bcl-2 family is classified as two groups, which is characterized by the presence of Bcl-2 homology (BH) domains: (1) anti-apoptotic proteins, containing the BH1–4 domains (Bcl-2, Bcl-XL, Bcl-w, and Mcl-1), and (2) pro-apoptotic proteins, containing the BH1–3 domains (Bax, Bak, and Bok), and BH3-only proteins, containing only BH3 domain (B-cell lymphoma 2 interacting mediator of cell death (Bim), p53 upregulated modulator of apoptosis (PUMA), and NOXA).1, 2 The balance of expression of the Bcl-2 family is important for the modulation of apoptosis in cancer cells.