MCL1 and cancer: Further, while being effective in cancers that depend on BCL2 for survival, such as chronic lymphocytic leukemia (CLL), resistance to ABT-199, as well as other inhibitors of BCL2, can occur through upregulation of other anti-apoptotic proteins such as BCL-XL, BFL1 (BCL2A1), or MCL1 [4,5].