LGR5 and breast cancer: Wnt signaling plays a functional role in epithelial stem cell biology, and has been shown to be critical for early stages of mammary development and breast cancer.1–4 Wnt signaling has also been suggested to play a role in MaSCs.5–8 Lgr5, a G-protein-coupled receptor involved in canonical Wnt signaling, was proposed as a marker for adult MaSCs, but other reports do not support this conclusion.9–12