Point mutation Glu255Lys at the ATP binding site of the ABL kinase in bone marrow samples from patients with Ph⁺ ALL before treatment with STI571. A 166-bp region of ABL, which codes for the ATP binding site, was PCR-amplified using 2 primer pairs and cloned into TA-cloning vector. Sequencing of a total of 432 clones revealed the Glu255Lys mutation in 2 clones established from 2 different STI571-naive bone marrow cell samples. (A) DNA sequence of clones from the STI571-naive cells having the wild-type BCR-ABL sequence. The critical nucleotide (nt 910) is marked in bold. (B) DNA sequence of the 2 clones from the STI571-naive cells carrying the mutation Glu255Lys, which is caused by a G>A change at nt 910. The position of the mutation is underlined. (C) For comparison, clones that were derived from STI571-resistant cells from the same patients that were previously shown by direct DNA sequencing to have the Glu255Lys mutation were sequenced. The nucleotide change G>A at nt 910 was detectable in 10% to 30% of clones from STI571-resistant cells. The position of the mutation is underlined.