MCL1 is an anti-apoptotic protein in the BCL2 family and amplifies frequently in multiple human cancers. Myeloid cell leukemia sequence 1 (MCL1) is a potent anti-apoptotic protein that plays a critical role in cell survival and drug resistance in various cancers. MCL1 is a pivotal member of the anti-apoptotic BCL-2 family proteins. While a distinctive feature of MCL1 resides in its efficient ubiquitination and destruction, the deubiquitinase USP9X has been implicated in the preservation of MCL1 expression by removing the polyubiquitin chains.MCL1 is also known as a potential contributor to the resistance of BCL2 inhibitors, as well as various chemotherapeutic drugs. MCL1 is, therefore, regarded as a potential target for cancer therapy. The overexpression of MCL1 promotes cell survival and confers chemotherapeutic resistance.