Involved in the regulation of apoptosis versus cell survival, and in the maintenance of viability but not of proliferation. Mediates its effects by interactions with a number of other regulators of apoptosis. Isoform 1 inhibits apoptosis. Isoform 2 promotes apoptosis. Interacts with HIF3A (via C-terminus domain) (By similarity). Interacts with BAD, BOK, BIK and BMF (By similarity). Interacts with PMAIP1 (PubMed:17389404). Interacts with BBC3 (By similarity). Isoform 1 interacts with BAX, BAK1 and TPT1 (PubMed:10837489, PubMed:12149273, PubMed:15077116). Heterodimer of isoform 1 and isoform 2. Homodimers of isoform 1 or isoform 2 are not detected. Isoform 2 does not interact with pro-apoptotic BCL2-related proteins (PubMed:10837489). Interacts with RTL10/BOP (PubMed:23055042). Interacts with BCL2L11; may sequester BCL2L11 to prevent its pro-apoptotic activity (PubMed:10837489, PubMed:27013495, PubMed:17389404, PubMed:20562877). Interacts with GIMAP5 and HSPA8/HSC70; the interaction between HSPA8 and MCL1 is impaired in the absence of GIMAP5 (By similarity). Cytoplasmic, associated with mitochondria. Expression increases early during phorbol ester-induced differentiation along the monocyte/macrophage pathway in myeloid leukemia cell line ML-1. Rapidly up-regulated by CSF2 in ML-1 cells. Up-regulated by heat shock-induced differentiation. Expression increases early during retinoic acid-induced differentiation. Cleaved by CASP3 during apoptosis. In intact cells cleavage occurs preferentially after Asp-127, yielding a pro-apoptotic 28 kDa C-terminal fragment. Rapidly degraded in the absence of phosphorylation on Thr-163 in the PEST region. Phosphorylated on Ser-159, by GSK3, in response to IL3/interleukin-3 withdrawal. Phosphorylation at Ser-159 induces ubiquitination and proteasomal degradation, abrogating the anti-apoptotic activity. Treatment with taxol or okadaic acid induces phosphorylation on additional sites. Ubiquitinated. Ubiquitination is induced by phosphorylation at Ser-159. Belongs to the Bcl-2 family.