As a first experimental model of human breast cancer growth in vivo, we injected separate groups of highly immunodeficient female NOD/SCID/IL2rγ−/− (NSG) or NOD/RAG1−/−IL2rγ−/− (NRG) mice subcutaneously (on the back) with cells from two widely used human breast cancer cell lines: MDA-MB-231 and SUM-149 cells. This evidence concerns the gene RAG1 and breast cancer.