T-cell leukemia/lymphoma protein 1A (abbreviated for TCL1A) is a member of the TCL1 family. TCL1 protooncogene is expressed in CD3-CD4-CD8- precursor T cells and is extinguished at the CD4+CD8+ stage of thymocyte development. In B cells, TCL1 is first expressed in pro-B cells and remains high in naive mantle zone B cells of peripheral lymphoid tissues. The TCL1 protooncogene is overexpressed in many mature B cell lymphomas, especially from AIDS patients. It has been demonstrated that TCL1 is a powerful oncogene that, when overexpressed in both B and T cells, predominantly yields mature B cell lymphomas. All TCL1 isoforms bind to the Akt pleckstrin homology domain. Both in vitro and in vivo TCL1 increases Akt kinase activity and as a consequence enhances substrate phosphorylation. In vivo, TCL1 stabilizes the mitochondrial transmembrane potential and enhances cell proliferation and survival. It has been shown that TCL1 is a novel Akt kinase coactivator, which promotes Akt-induced cell survival and proliferation.