ABL1 and acute lymphoblastic leukemia: BCR/ABL1+“Stem/B” cells that selectively persist at remission are more quiescent (G0) and cycle less actively (S–M–G2) than leukemic “Pro-B” cells.[19] The speed of blast clearance during therapy is a major prognostic factor of the outcome in childhood ALL.[9,20–22] The blast counts in the BM on days 15 and 33 have been widely used to deliver risk-directed therapy.[21]