BRCA1 and breast cancer: With the help of single-cell RNA sequencing (scRNA-seq) technology, in one of our recent studies, we found that the heterogeneous subgroups within the BRCA1-deficient mammary tumors are classified mainly due to the different activities of cell proliferation, metabolism, DNA damage response (DDR) and epithelial-to-mesenchymal transition (EMT) (Figure 1A) 24.