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.