Human lymphocytes resistant to 6-thioguanine increase with age

Using an autoradiographic technique we determined the number of circulating lymphocytes that were resistant to 6-thioguanine and which were presumably mutants at the hypoxanthine--guanine phosphoribosyl transferase locus. The number in normal individuals was found to increase exponentially with age. The data suggest a relationship between mutagenesis and ageing, perhaps by way of a decline with age in the fidelity of DNA replication or repair.