Formation and repair of O6-methylguanine and methylation of the Ha-ras proto-oncogene by N-methyl-N-nitrosourea are not associated with mammary tumor resistance in the Copenhagen rat

A high incidence of mammary adenocarcinomas is induced in sexually immature, female Buf/N rats by a single dose of N-methyl-N-nitrosourea (MNU). The Ha-ras gene is activated in a majority of these tumors. The Copenhagen (Cop) rat is completely resistant to mammary gland carcinogenesis by a number of carcinogens, including MNU. Here we show that MNU-treated Buf/N and Cop rats do not differ in the extent of formation and rate of repair of O 6 -methylguanine in DNA isolated from their mammary epithelial cells