Methylation analysis of 2 cases of CML in which the translocation breakpoint is within the EcoRI flank. In both cases (CML-A and -B), EcoRI restriction alone (lanes labeled E) results in two distinct bands: a normal band at 6.2 kb and a rearranged band of higher molecular weight (indicated by a star). In both cases, after digestion with Not I (EN lanes), the unrearranged allele is completely digested to a 5.4-kb band, indicating complete lack of methylation. The rearranged allele does not change in size afterNot I digestion, which could be due either to complete methylation or to a breakpoint upstream of the Not I site. For comparison purposes, normal DNA digested with EcoRI alone is shown in lane 1.