Tumor cell lines are important tools for the study of cancer. However, most tumor cell lines available today do not mimic physiological tumor development, progression, and host immune responses. Autochthonous tumors include spontaneously occurring tumors and chemical, viral, or physical carcinogen-induced tumors. They are considered to model human tumors more closely than transplanted tumors. Autochthonous tumors can be generated de novo in a model organism of interest and are thought to resemble physiological human tumor conditions. There is therefore a need for development of cell lines from autochthonous tumor models that recapitulate physiological tumor conditions to provide relevant tools for studying tumor development and progression, and host immune responses to cancers. Researchers at the National Cancer Institute (NCI) have developed a mouse breast tumor cell line, AT-3, derived from cells of the primary mammary gland carcinoma of the MTAG model. The MTAG mouse model was developed using the mouse mammary tumor virus (MMTV) long terminal repeat (LTR) promoter to specifically target the polyomavirus middle T antigen (Ag) expression in the mammary gland tissue of B6 mice. This strategy resulted in the generation of transgenic mice with autochthonous mammary carcinomas, with eventual metastatic spread to the lungs that occurs over an approximate 6-month life span. The AT-3 cell line has further been used to study tumor-specific immune dysfunction in the B6 mouse background. NCI is seeking licensees for the AT-3 mouse breast tumor cell line.