LGR4 and breast cancer: As an example, for breast cancer-specific organoids, the epithelial growth factor receptor (EGFR) ligand EGF to promote cell proliferation, the bone morphogenetic protein (BMP) inhibitor Noggin for cellular expansion, and LGR4/5 (leucine-rich repeat-containing G-protein-coupled receptor 4/5) ligand R-spondin, which acts as an agonist of the canonical Wnt/β-catenin signaling pathway are commonly used culture systems [22,35,36,37].