Three major stem cell signaling pathways (Notch, Wnt, and Hedgehog) and other critical cellular signaling pathways (estrogen receptor, PI3K, MAPK, JAK/STAT, NFκB, and TGF-β) participate both in normal mammary gland development and in breast cancer and cancer stem cells [91, 92]. The gene discussed is ESR1; the disease is breast carcinoma.