The p210 form of BCR–ABL is seen in 95% of all patients with CML and up to 20% of adult patients with acute lymphocytic leukaemia (ALL) (Bartram et al, 1983; Shtivelman et al, 1985), whereas the p185 form is seen in 5–10% of de novo ALL (Hermans et al, 1987; Clark et al, 1988). This evidence concerns the gene ABL1 and acute lymphoblastic leukemia.