The Brca1co/coMMTV-Cre model offers a uniquely relevant in vivo platform for preclinical drug development by effectively reproducing the key hallmarks of human BRCA1-associated mammary tumors, including basal-like histology, dominant blood-derived macrophage infiltration, low baseline TIL density, and sensitivity to both DNA damage and immunotherapy. This evidence concerns the gene BRCA1 and breast cancer.