Structural Biology Developing drugs that target a specific subtype in a G protein–coupled receptor (GPCR) family is a major challenge. Maeda et al. examined the basis of specificity of a snake venom toxin binding to muscarinic acetylcholine receptors (MAChRs), which mediate many functions of the central and parasympathetic nervous systems. They determined a structure that shows why the mamba venom toxin MT7 is specific for one receptor, M1AChR, and also explains how it inhibits downstream signaling. Based on this structure, they engineered MT7 to be selective for another receptor, M2AChR, instead of M1ChR. The toxin may present a promising scaffold for developing specific GPCR modulators. Science , this issue p. [161][1] [1]: /lookup/doi/10.1126/science.aax2517