In Hartnup disease, an amino acid transport disorder caused by variations in SLC6A19, the gene responsible for encoding the transporter of neutral epithelial amino acids B(0)AT1, single nucleotide polymorphisms (SNPs) have been identified; they decrease the transport activity when expressed alone or co-expressed with the angiotensin converting enzyme 2 (ACE2); however, these SNPs are characteristic of the presence of this disease [68]. The gene discussed is ACE2; the disease is Hartnup disease.