TCL1 (T cell leukemia 1), MTCP1 and TCL1b belong to the TCL1 proto-oncogene family, and their products are involved in Akt activation during embryonic development, T cell leukemias, prolymphocytic leukemias and B cell lymphomas. The Akt association domain of TCL1 binds with the PH domain of Akt. The formation of an oligomeric TCL-Akt complex is required for TCL1 coactivator function and results in phosphorylation and activation of Akt . Furthermore, functional analysis indicates that the interaction between TCL1 and Akt promotes translocation of Akt to the nucleus. These findings are supported by the crystal structure of TCL1, which suggests that TCL1 may participate in molecular transport.