Bartter syndrome refers to a group of disorders that are unified by autosomal recessive transmission of impaired salt reabsorption in the thick ascending loop of Henle with pronounced salt wasting, hypokalemic metabolic alkalosis, and hypercalciuria. Clinical disease results from defective renal reabsorption of sodium chloride in the thick ascending limb (TAL) of the Henle loop, where 30% of filtered salt is normally reabsorbed (Simon et al., 1997). Patients with antenatal (or neonatal) forms of Bartter syndrome (e.g., BARTS1, 601678) typically present with premature birth associated with polyhydramnios and low birth weight and may develop life-threatening dehydration in the neonatal period. Patients with classic Bartter syndrome present later in life and may be sporadically asymptomatic or mildly symptomatic (summary by Simon et al., 1996 and Fremont and Chan, 2012). Genetic Heterogeneity of Bartter Syndrome Antenatal Bartter syndrome type 1 (601678) is caused by loss-of-function mutations in the butmetanide-sensitive Na-K-2Cl cotransporter NKCC2 (SLC12A1; 600839). Antenatal Bartter syndrome type 2 (241200) is caused by loss-of-function mutations in the ATP-sensitive potassium channel ROMK (KCNJ1; 600359). One form of neonatal Bartter syndrome with sensorineural deafness, Bartter syndrome type 4A (602522), is caused by mutation in the BSND gene (606412). Another form of neonatal Bartter syndrome with sensorineural deafness, Bartter syndrome type 4B (613090), is caused by simultaneous mutation in both the CLCNKA (602024) and CLCNKB (602023) genes. Also see autosomal dominant hypocalcemia-1 with Bartter syndrome (601198), which is sometimes referred to as Bartter syndrome type 5 (Fremont and Chan, 2012), caused by mutation in the CASR gene (601199). See Gitelman syndrome (GTLMN; 263800), which is often referred to as a mild variant of Bartter syndrome, caused by mutation in the thiazide-sensitive sodium-chloride cotransporter SLC12A3 (600968).