Inwardly rectifying K⁺ (K_ir) channels have diverse physiological functions depending on their type and their location. To date, 15 K_ir subunit genes have been identified and classified into seven subfamilies (K_ir1 to K_ir7). These subfamilies can be categorized into four functional groups: 1) classical K_ir channels (K_ir2.x), 2) G-protein gated K_ir channels (K_ir3.x), 3) ATP-sensitive K⁺ channels (K_ir6.x), and 4) K⁺-transport channels (K_ir1.x, K_ir4.x, K_ir5.x, and K_ir7.x).