To investigate the relationship between the pattern of methylation at the major breakpoint cluster region(M-BCR) and transformation of chronic myelocytic leukemia(CML) from the chronic to the blastic phase,the M-BCR methylation status was examined serially from chronic to blastic phase in 23 CML patients.The DNA of mononuclear cells from bone marrow or peripheral blood was digested with restriction enzymes HpaⅡand BgⅢ,and hybridized with a 5’ M-BCR probe.The methylation status was stable during evolution of CML from chronic to the myeloid blastic phase.Cells in both phases showed consistent methylation patterns consisting of fully methylated rearranged fragments of variable size,4.8,3.1/3.0,and 2.7/2.5 kb.Conversely,there was substantial heterogeneity in methylation patterns in patients with lymphoid crisis.All lymphoid-crisis patients studied in blastic phase showed a pattern distinct from that of the chronic phase in the same patient,as well as from the myeloid pattern,suggesting cell lineage-specific M-BCR methylation.Moreover,in four of six patients with lymphoid crisis,the chronic-phase patterns were different from those of cases with myeloid crises.Ph-positive and -negative acute lymphocytic leukemia(ALL) showed methylation patterns different from those of lymphoid crisis in CML.Although the number of patients with lymphoid crisis studied has been limited,these results suggest that analysis of M-BCR methylation status may be of clinical use in distinguishing lymphoid from myeloid crises and predicting the cell lineage of a crisis when the disease is still in the chronic phase.(C) 2001 Elsevier Science Inc.All rights reserved. To investigate the relationship between the pattern of methylation at the major breakpoint cluster region (M-BCR) and transformation of chronic myelocytic leukemia (CML) from the chronic to the blastic phase, the M-BCR methylation status was examined serially from chronic to blastic phase in 23 CML patients. The DNA of mononuclear cells from bone marrow or peripheral blood was digested with restriction enzymes HpaII and BgIII, and hybridized with a 5 ’M-BCR probe. The methylation status was stable during evolution of CML from chronic to the myeloid blastic phase. Cells in both phases showed consistent methylation patterns consisting of fully methylated rearranged fragments of variable size, 4.8, 3.1 / 3.0, and 2.7 / 2.5 kb. Conversely, there was substantial heterogeneity in methylation patterns in patients with lymphoid crisis. -crisis patients studied in blastic phase showed a pattern distinct from that of the chronic phase in the same patient, as well as from the myeloid pattern, suggesting cell lineage-specific M-BCR methylation.Moreover, in four of six patients with lymphoid crisis, the chronic-phase patterns were different from those of with myeloid crises. Ph-positive and -negative acute lymphocytic leukemia (ALL) showed methylation patterns different from those of lymphoid crisis in CML .Although the number of patients with lymphoid crisis has been been limited, these results suggest that analysis of M-BCR methylation status may be of clinical use in distinguishing lymphoid from myeloid crises and predicting the cell lineage of a crisis when the disease is still in the chronic phase. (C) 2001 Elsevier Science Inc. All Rights Reserved.