Due to the proposed conservation of structure and function between p13 MTCP1 and p14 TCL1A, the fungible role of both in T-PLL pathogenesis, the known role of TCL1A in CLL29, identification of Xq28 translocations in CLL with unresolved significance, and the apparent influence of MTCP1 expression on CLL outcomes, we hypothesized MTCP1 acts as a leukemogenic co-stimulator—revealing an unrecognized factor in CLL pathogenesis. The gene discussed is TCL1A; the disease is B-cell chronic lymphocytic leukemia.