MLL gene rearrangements are retained in cultured leukemic lymphoblasts. DNA was extracted from cells of case no. 1 before culture and from a clone initiated by a single leukemic cell after 3 months of culture. DNA was digested with the restriction enzymes indicated and probed with an MLL cDNA probe. Arrows indicate the rearranged bands; G corresponds to the germline bands. The KB epidermoid carcinoma line, which has normal MLL alleles, was used as a control.