The study by Chen et al. investigates the mechanism by which the steroidal alkaloid cyclopamine inhibits Hedgehog (Hh) signaling, a process that is both teratogenic and antitumor. The authors use photoaffinity and fluorescent derivatives of cyclopamine to demonstrate that the inhibitory effect is mediated by direct binding to the heptahelical bundle of Smoothened (Smo). This binding likely influences the conformation of Smo, and cyclopamine can reverse the retention of partially misfolded Smo in the endoplasmic reticulum. The study also shows that the binding of cyclopamine to Smo is sensitive to the function of Patched (Ptc), suggesting that Ptc may modulate Smo structure. These findings provide a molecular basis for the teratogenic and antitumor activities of cyclopamine and suggest that small molecules can regulate Smo activity, potentially through endogenous ligands. The results have implications for the therapeutic use of Smo antagonists in treating Hh-related diseases, such as medulloblastoma and basal cell carcinoma.The study by Chen et al. investigates the mechanism by which the steroidal alkaloid cyclopamine inhibits Hedgehog (Hh) signaling, a process that is both teratogenic and antitumor. The authors use photoaffinity and fluorescent derivatives of cyclopamine to demonstrate that the inhibitory effect is mediated by direct binding to the heptahelical bundle of Smoothened (Smo). This binding likely influences the conformation of Smo, and cyclopamine can reverse the retention of partially misfolded Smo in the endoplasmic reticulum. The study also shows that the binding of cyclopamine to Smo is sensitive to the function of Patched (Ptc), suggesting that Ptc may modulate Smo structure. These findings provide a molecular basis for the teratogenic and antitumor activities of cyclopamine and suggest that small molecules can regulate Smo activity, potentially through endogenous ligands. The results have implications for the therapeutic use of Smo antagonists in treating Hh-related diseases, such as medulloblastoma and basal cell carcinoma.