Unusually among ATP-binding cassette proteins, the sulfonylurea receptor (SUR) acts as a channel regulator. ATP-sensitive potassium channels are octameric complexes composed of four pore-forming subunits and four regulatory SUR subunits. Two different genes encode SUR1 (ABCC8) and SUR2 (ABCC9), with the latter being differentially spliced to give SUR2A and SUR2B, which differ only in their C-terminal 42 amino acids.