BCR and chronic myelogenous leukemia, BCR-ABL1 positive: Although ectopic PTP1B expression impairs clonogenicity and tumorigenesis of p210-BCR/ABL-transformed Rat-1 fibroblasts and induces K562 erythroid maturation (LaMontagne et al, 1998); it is still unclear why PTP1B expression is increased in BCR/ABL-expressing cells (LaMontagne et al, 1998) and whether PTP1B activity is altered in primary CML progenitors or changed during disease progression.