The Philadelphia chromosome is present in less than 5% of children with acute lymphoblastic leukemia (ALL), but in approximately 30% of adult cases and in more than 50% of ALL patients over the age of 50 [1]. The Philadelphia chromosome results from the reciprocal translocation which creates a hybrid BCR-ABL gene. A 210 kD BCR-ABL fusion protein (p210) occurs in an overwhelming proportion of patients with chronic myelogenous leukemia (CML) but in less than a third of those with ALL. A shorter 190 kD BCR-ABL fusion protein is present in 50% of adult Ph+ ALL patients and in 80% of childhood Ph+ ALL cases [2].