A more recent classification system has been developed, which designates patients who are homozygous for the SLC3A1 mutations as cystinuria type A, patients who are homozygous for the SLC7A9 mutations as type B, and those who have a mutation in both the SLC3A1 and SLC7A9 genes as type AB.31 Here, SLC7A9 is linked to cystinuria.