In some cases tumors induced by non-acute retroviruses have shown evidence for activation of more than one proto-oncogene in the same tumor—e.g., int-1 (Wnt-1) and int-2 in MMTV-induced mammary tumors [72] and pim-1 and c-myc in M-MuLV-induced tumors [46]. This evidence concerns the gene WNT1 and breast cancer.