Besides recurrent inactivating mutations of the cell cycle inhibitor CDKN1Bb (p27) in 16% of patients postulated to help BRAF V600E-mutated malignancies escape cell senescence [24], other identified inactivating gene mutations suggest an even more heterogeneous pattern of genetic alterations, each being detectable only in a small subgroup of HCL patients [23]. Here, CDKN1B is linked to hairy cell leukemia.