sentence: K channels form as 4 +4 octamers of Kir 6.x pore-forming subunits and proteins.97 Two Kir6 subunits , Kir6.1 and 6.2 , have been identified , and two SUR genes are known , SUR1 and SUR2 , the latter giving rise to SUR2A and SUR2B by alternative splicing .