The Bcl-2 family of proteins plays a pivotal role in determining the life and death of a cell. Members of the Bcl-2 family all possess at least one of four conserved Bcl-2 homology domains (BH1-BH4) which mediate protein-protein interactions. They can be divided into molecules that have either an anti-apoptotic or pro-apoptotic function. The ratio between these two groups of molecules can help determine whether a cell lives or dies. The majority of anti-apoptotic family members contain common BH1-BH4 domains and are most similar to Bcl-2. Mcl-1 is a member of the anti-apoptotic Bcl-2 group of proteins, including Bcl-x, Bcl-w, A1/Bfl-1, Boo/Diva, and Nr-13. Mcl-1 was cloned from the ML-1 human myeoblastic leukemia cell line. Mcl-1 localizes to the mitochrondria, like Bcl-2, but also associates with membranes via its C-terminal hydrophobic domain. Further studies have shown that Mcl-1 is upregulated in response to the GM-CSF and IL-3 cytokine signaling pathways and that this regulation of Mcl-1 is controlled at the level of transcription. Mcl-1 is detected at approximately 40 kDa in SDS/PAGE.