The hedgehog signaling pathway, which plays key roles during embryonic development, is involved in growth and proliferation of multiple kinds of cancer. Blocking unwanted activation of this signaling pathway is an important anticancer target. Although several inhibitors of downstream members of the pathway have been identified, so far the only known inhibitor of the first step in the pathway is a modestly potent molecule called robotnikinin. Now, a team led by Rudi Fasan of the University of Rochester has designed a macrocyclic peptide that more effectively blocks the first step in the pathway, the binding of a signaling protein called sonic hedgehog to a transmembrane receptor called patched (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b06087). The researchers based the peptide on a loop in hedgehog-interacting protein, which suppresses the signaling pathway. They swapped a leucine for a cysteine and replaced a methionine with a nonnatural amino acid that forms a thioether bridge with the cysteine. Then they made a library in which they changed five other amino acids in the 13-amino acid peptide and screened for hedgehog inhibitors. They identified a peptide that suppresses activation of the hedgehog signaling pathway in living cells with nanomolar potency. In addition, the inhibitor also blocks other hedgehog analogs.