SLC7A9 and cystinuria: In particular, the observation that AGT1 serves as the second partner of the rBAT transporter in the S3 segment and its expression is consistent with the inversed expression of the so far known cystine transporter subunits SLC3A1/rBAT and SLC7A9/ b0,+AT [8] strongly points to SLC7A13 as a candidate gene for cystinuria.