For instance, int-2/fibroblast growth factor-3 (FGF3), an oncogene that is recurrently amplified in human breast tumors, first came to prominence because of its dysregulation resulting from a shared integration site of mouse mammary tumor virus in murine models of breast cancer (Nusse and Varmus, 1982; Casey et al., 1986; Callahan, 1996). This evidence concerns the gene FGF3 and breast cancer.