SLC7A9 encodes the light subunit of a heterodimeric amino acid transporter complex that associates with SLC3A1 to mediate electrogenic exchange of cationic amino acids and L-cystine for neutral amino acids 1. Operating as a system b(0,+)-like transporter, SLC7A9 demonstrates high affinity for extracellular cationic amino acids and L-cystine, with substrate exchange driven by intracellular neutral amino acids and reduction of L-cystine to L-cysteine 2. The transporter is essential for reabsorption of L-cystine and dibasic amino acids across the renal proximal tubule brush border membrane 3. SLC7A9 mutations are the primary cause of cystinuria, the most common genetic cause of pediatric nephrolithiasis, accounting for approximately 15% of monogenic kidney stone disease 4. Loss-of-function mutations in SLC7A9 prevent cystine reabsorption, leading to elevated urinary cystine excretion and formation of poorly soluble cystine calculi 3. Beyond renal physiology, SLC7A9 overexpression in gastric cancer promotes cell proliferation and chemotherapy resistance by facilitating cystine import and suppressing ferroptosis, independent of SLC7A11 5. Clinical management of SLC7A9-related cystinuria focuses on reducing cystine excretion and increasing solubility through hydration, dietary modification, urinary alkalinization, and thiol-binding drugs 6.