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.