Mediates the transport of urea driven by a concentration gradient across the cell membrane of erythrocytes (PubMed:7989337, PubMed:10514515, PubMed:7797558, PubMed:8997401). Also mediates the transport of urea across the cell membrane of the renal inner medullary collecting duct which is critical to the urinary concentrating mechanism (By similarity). Facilitates water transport in erythrocytes (By similarity). urea(in) = urea(out) Inhibited by phloretin and para-chloromercuribenzene sulfonate. Homotrimer; each subunit contains a pore through which urea permeates (By similarity). Identified in a complex with STOM (PubMed:23219802). Restricted to the basolateral membrane in various portions of the urothelium. Detected in erythrocytes (at protein level) (PubMed:23219802). Expressed in spleen erythroblasts and tumoral kidney (PubMed:7989337). SLC14A1 is responsible for the Kidd blood group system (JK) [MIM:111000]. JK is defined by 2 alleles, JK*01 and JK*02 that give rise to Jk(a) and Jk(b) antigens respectively. The molecular basis of the Jk(a)/Jk(b) antigens is a single variation in position 280; Asp-280 corresponds to Jk(a) and Asn-280 to Jk(b). Some individuals carry silenced JK*01 and JK*02 alleles, designated JK*01N or JK*02N. They results in a Jk(null) phenotype associated with reduced urea permeability of red blood cells. Jk(null) is not associated with any obvious clinical syndrome except for a urine concentration defect. Belongs to the urea transporter family. Blood group antigen gene mutation database