The amino acid transporter System b0,+ is expressed at the apical membrane of the renal proximal tubule. System b0,+ is a Na+-independent antiporter mediating dibasic amino acid and cystine influx in exchange for neutral amino acid efflux. System b0,+ consists of two membrane proteins, rBAT and b0,+AT, encoded by the genes SLC3A1 and SLC7A9, respectively [1]. Mutations are associated with cystinuria with a recessive inheritance pattern in SLC3A1 and a variable penetrance of phenotype in SLC7A9. Cystinuria is characterised by reduced reabsorption of dibasic amino acids and cystine, an aminoaciduria, cystine precipitation and the formation of renal calculi. In this study, mutations in SLC3A1 and/or SLC7A9 were identified in a UK cohort of cystinuric patients. A novel mutation, Y579D, was identified in SLC3A1 of two unrelated patients who were compound heterozygotes for Y579D and the known SLC3A1 mutations M467T or R452W. The inheritance pattern of these mutations was established through segregation analysis. The human SLC3A1 cDNA was mutated in vitro to introduce Y579D. rBAT-Y579D cRNA was synthesised and injected (1-50ng) into Xenopus laevis oocytes to determine expression and function of System b0,+. Results were compared with those using wild-type human rBAT and rBAT-M618I, an SNP found in 46% of alleles in the general population and 42% of this cohort. System b0,+ activity was measured by [3H]arginine (2.5µCi/ml, 10µM) uptake (60min, 22-24°C) in cRNA-injected oocytes (1-6 days post-injection). [3H]Arginine uptake was similar in oocytes injected with either rBAT or M618I. In contrast, there was a marked reduction in [3H]arginine uptake in Y579D-injected oocytes under most conditions. On day 1 post-injection, there was a significant (p<0.01) increase in uptake in rBAT-injected (50ng) oocytes, uptake being 5.7-fold greater than control (water-injected oocytes), whereas in Y579D-injected oocytes uptake was not significantly (p>0.05) different from control. [3H]Arginine uptake in Y579D-injected oocytes was dependent upon the quantity of cRNA injected and the number of days post-injection. Thus recovery of Y579D function was observed at day 6 post-injection but only in oocytes injected with 50ng cRNA. Immunoblotting and immunocytochemical measurements indicate that Y579D protein is synthesised within the oocytes but less is expressed at the plasma membrane suggesting that the decrease in function observed with Y579D is due to less efficient processing and trafficking of System b0,+. Similar observations have been made with the most common SLC3A1 missense mutation, M467T, which causes misfolding, abnormal trafficking, and reduced membrane expression of System b0,+ [2]. In the long term, the identification of the molecular cause and functional consequences of monogenic disorders such as cystinuria should aid in the development of personalised patient treatment.