GIRK4 immunoreactivity colocalizes with immunoreactivity for HCN4 (as a marker for pacemaker cells) and GIRK1 in cells from the SAN of wild-type mice; GIRK4 immunoreactivity is lost and GIRK1 fails to localize at the membrane in cells from Girk4−/− mice. (From Mesirca et al., 2013.)