Mammary tumors in feral mice lacking MuMTV DNA.

Two mammary tumors developed in feral mice treated with dimethylbenzanthracene. The tumors, livers, and spleens of these animals contained no MuMTV proviral DNA. Neither tumor had amplified or rearranged int-1 or int-2 loci. This demonstrates that mammary tumorigenesis can occur in mice in the absence of endogenous and exogenous MuMTV.