BRCA1 and breast cancer: To understand the genetic features and evolution of BRCA1-deficient breast cancers and identify their drivers, we perform both bulk tissue WES and single-cell WES (scWES) for BRCA1-deficient mammary glands and tumors, followed by validation with Sanger sequencing or droplet digital PCR (ddPCR) assays, as well as functional validation with clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated protein 9 (Cas9)-mediated knockouts in vitro and in vivo.