SLC7A9, also known as B0, +- type amino acid transporter (B0, +AT), is a member of the cationic amino acid transporter/glycoprotein-associated family, and it functions as a heterodimer with the amino acid transport protein SLC3A1/rBAT. The SLC7A9 protein is predominantly distributed in the kidney, small intestine, liver, and placenta. Defective mutations in the SLC7A9 and/or SLC3A1 gene cause cystinuria, which is characterized by impaired transport of cystine and dibasic amino acids through the epithelial cells of the renal tubule and gastrointestinal tract; this results in the formation of calculi due to cystine precipitation in the urinary tract, leading to obstruction, infections, and ultimately, renal insufficiency.