MCL1 and cancer: BCL2, MCL-1 and BCL-XL, the main anti-apoptotic proteins, were identified as cellular oncogenes, as they are frequently found to be overexpressed in human cancers and prevent normal or protective cell death mechanisms; therefore, there is a focus on developing inhibitors that have a high affinity and relative specificity for one or more members of this protein family [27,28,29,30].