The int-1 proto-oncogene is the first cellular gene discovered and implicated in tumorigenesis solely on the basis of repeated insertional mutations that activate transcription of the gene. The gene is silent in most tissues but expressed in the embryonic central nervous system, in the late (post-meiotic) stages of spermatogenesis, and in a high proportion of mouse mammary tumor virus-induced carcinomas, when a provirus is inserted upstream or downstream of the coding domain. The functional significance of int-1 in the oncogenic process is supported by the demonstration that murine leukemia virus-based vectors carrying the gene can alter the morphology and growth properties of an established line of mammary epithelial cells. The predicted primary protein product of the int-1 gene is 370 amino acids in length and cysteine-rich; immunoprecipitation with anti-peptide antibodies reveals multiple species of int-1 protein, due to asparagine-linked glycosylations and probable cleavage of a signal peptide. However, the active product of the gene and its biochemical behavior during normal development and mammary tumorigenesis are not known.