SLC7A9 as a heterodimer with SLC3A1 in the plasma membrane mediates the exchange of arginine (L-Arg), lysine (L-Lys), or cystine (CySS-) for leucine (L-Leu) and other neutral amino acids. The physiological concentrations of these amino acids favor neutral amino acid export and arginine/lysine/cystine import. Defects in SLC7A9 and SLC3A1 cause cystinuria. In the body, this transport process is prominent in the kidney (Mizoguchi et al. 2001).