α-Conotoxin SIA is a selective nicotinic acetylcholine receptor (nAChR) antagonist with high affinity for the muscle nAChR. α-Conotoxin SIA preferentially targets the α/δ interface of the muscle nAChR in mouse muscle. In contrast, for Torpedo nAChR, α-Conotoxin SIA has a much higher affinity for the α/γ than for the α/δ interface.