SLC12A1 and Bloom syndrome: At present, BS is classified according to the causative gene (7): BS1, with SLC12A1 encoding the sodium-potassium-chloride cotransporter in thick ascending limb of the loop of Henle; BS2, with KCNJ1 of the luminal K+ channel ROMK; BS3, with CLCNKB of the basolateral chloride channel CIC-Kb; BS4a, with BSND encoding the beta-subunit for CIC-Ka and CIC-Kb; or BS4b, with simultaneous mutations in CLCNKA and CLCNKB (8–11).