Deletions in 12p13 that lead to haplo-insufficiency of the CDKN1B gene have been suggested to play a role in the pathogenesis of T-PLL, as this gene encodes the cyclin-dependent kinase inhibitory protein p27 (KIP1), which functions in cell-cycle regulation [43, 44]. Here, CDKN1B is linked to T-cell prolymphocytic leukemia.