ABL1 and acute lymphoblastic leukemia: Likewise, in acute lymphocytic leukemia patients, which lack the BCR/ABL fusion protein, HCK expression is reduced due to aberrant HCK gene methylation [64], while in acute promyelocytic leukemia, HCK transcription is repressed by the fusion protein promyelocytic leukemia-retinoic acid receptor α (PML-RARα).