Different aspects related to initiation of chronic myelogenous leukemia by the t(9;22) translocation and progression of the disease were investigated. Computer search indicated that the repeat within BCR exon I has significant sequence homology to the long terminal repeats of three retroviruses, to two transposons and to the Alu family. This raises the possibility that the BCR repeat is involved in the t(9;22) as well as in generation of the BCR-related loci. Possible involvement of the p53 gene in clinical transition to acute phase was studied. In six patients and cell lines where one allele of the gene was deleted, the other allele was inactivated by loss of transcription, point mutation or rearrangement. The majority of patients, however, have both p53 alleles; detailed analysis of the p53 gene in several of them indicated normal transcription and amino acid sequence.