Although much is known about the consequences of the BCR‐ABL fusion protein in CML, the function of BCR and ABL in intact animals is not well understood (Schwartzberg et al, 1989; Tybulewicz et al, 1991; Voncken et al, 1996). Here, ABL1 is linked to chronic myelogenous leukemia, BCR-ABL1 positive.