On the theory of clonal selection in carcinogenic transformation.

A temperature sensitive variant (ts13) of African green monkey kidney cell line, carrying a temperature sensitive lesion in thymidine metabolism, was transformed by 40 μg/ml of methylnitrosourea (MNU) at the restricted temperature of 39·5°C, whereas the same cell line was not transformed by MNU at the permissive temperature of 33°C. Results presented in this paper raise the possibility that clones of cells carrying a biochemical lesion might possibly contribute to carcinogenesis with respect to certain chemical carcinogens.