BRCA1 and breast cancer: Single-cell RNA sequencing of tumors from mouse mammary tumor virus (MMTV)-polyoma middle tumor-antigen (PyMT), MMTV-Neu, and BRCA1-null murine models of breast cancer was also used to demonstrate that these breast tumors consist of multiple breast cancer subtypes (12) that include epithelial cells and pockets of invasive mesenchymal-like cells.