Bcl-X is one of the members of Bcl-2 family proteins, and exists in several isoforms generated by alternative splicing and plays a critical role in the control of apoptosis. The large isoform Bcl-XL (30KD)contains the highly conserved BH1 and BH2 amino acid domains and protects cells against apoptosis, whereas the short isoform, Bcl-XS(18KD) which deletes the conserved BH1/BH2 domain by alternative mRNA splicing, promotes apoptosisby interacting with Bcl-XL and Bcl-2.Bcl-X(S) is expressed at high levels in cells that undergo a high rate of turnover, such as developing lymphocytes. In contrast, Bcl-X(L) is found in tissues containing long-lived postmitotic cells, such as adult brain.Clone 2H12 has been shown to be useful for western blotting and immunoprecipitation, immunohistochemistry, and immunofluorescence of the Human, Mouse and Rat Bcl-XL and Bcl-XS proteins.