O6-Methylguanine removal by competent and incompetent human lymphoblastoid lines from the same male individual.

Two cell lines, one proficient (Mex+) and one deficient (Mex-) in the ability to remove O6-methylguanine, have been isolated by Epstein-Barr virus-mediated transformation of a single blood sample obtained from a normal human male. Extracts of untreated cells differ in their O6-methylguanine transferase (methyl acceptor protein) activity. Although both lines arise from the same individual, they show great difference in their sensitivities to the toxic action of N-methyl-N'-nitro-N-nitrosoguanidine. Chromosome counts of the strains reveal a modal number of 46 for both. Neither X-inactivation nor a gross abnormality in chromosome number can be the cause of the difference between the two lines.