ABL1 and acute lymphoblastic leukemia: Clinically, homozygous del(9p21) is predominantly found in T‐ALL patients and particularly in paediatric cases, in which the deletion confers poor prognosis.62, 63 The del(9p21) in conjunction with t(4;11) or other KMT2A translocations are seen but at a lower frequency than in other cytogenetic subgroups such as t(1;19) and t(9;22)/BCR‐ABL, indicating that the inactivation of CDKN2A/2B is not indispensable for the malignant phenotype.64