The cells of origin of BRCA1-associated mammary tumours, the mammary luminal epithelial estrogen receptor (ESR1)-negative stem/progenitor population, express the c-KIT (or KIT) receptor tyrosine kinase at high levels, as well as its downstream pathway member, the SRC-family kinase LYN (Regan et al., 2012; Tornillo et al., 2018). The gene discussed is ESR1; the disease is breast cancer.