We have found that PECAM-1 was heavily tyrosine phosphorylated in all the four Ph+ leukemic blast samples we examined, including those from biphenotypic AL, ALL and CML-BC patients, the effect was drastically inhibited by imatinib, thus indicating its phosphorylation was mediated by BCR/ABL (Fig. 1). This evidence concerns the gene ABL1 and chronic myelogenous leukemia, BCR-ABL1 positive.