SLC7A9 and cystinuria: Accordingly, cystinuria is divided into the following subtypes: biallelic SLC3A1 mutations (AA genotype), biallelic SLC7A9 mutations (BB genotype), a single heterozygous SLC3A1 mutation (A genotype), a single heterozygous SLC7A9 mutation (B genotype), and a single heterozygous SLC3A1 mutation combined with a single heterozygous SLC7A9 mutation (AB genotype) (Dello Strologo et al., 2002; Sahota et al., 2019).