BCL2L3, also known as MCL1(myeloid cell leukemia sequence 1) encodes an anti-apoptotic protein, which is a member of the Bcl-2 family. Alternative splicing results in multiple transcript variants. The longest gene product (isoform 1) enhances cell survival by inhibiting apoptosis while the alternatively spliced shorter gene products (isoform 2 and isoform 3) promote apoptosis and are death-inducing.Using the methods of somatic cell hybrid analysis and fluorescence in situ hybridization), the MCL1 gene is mapped to human 1q21.MCL1is a critical and specific regulator essential for ensuring the homeostasis of early hematopoietic progenitors.Phosphorylation of MCL1 directs its interaction with the tumor suppressor protein FBW7, which is the substrate-binding component of a ubiquitin ligase complex. The polyubiquitylation of MCL1 then targets it for proteasomal degradation. The degradation of MCL1 was blocked in patient-derived tumor cells that lacked FBW7 or had loss-of-function mutations in FBW7, conferring resistance to antitubulin agents and promoting chemotherapeutic-induced polyploidy.