A characteristic DNA rearrangement, the loss of an EcoRI cleavage site next to the 3'-end of the human c-mos gene, has been found to be frequently present in DNA from transformed hematopoietic cells of the myeloid lineage but not in DNA from either normal or transformed cells of different tissue types. Three established cell lines, respectively a pro-monocytic line (CM-S) and two precursor granulocytic lines (My/K1 and My/K5), carry the same genome rearrangement, but not fibroblasts obtained from the marrow of the same patients. This DNA rearrangement is maintained in three different hybridomas derived by fusion of CM-S cells with normal human embryo hepatocytes.