SLC6A19 and Hartnup disease: Indeed, homozygous or compound heterozygous mutations in the neutral amino acid transporter SLC6A19 (Table 1), which is required for tryptophan transport, cause a pellagra‐like disorder known as Hartnup disease (Kleta et al,2004; Seow et al,2004; Cheon et al,2010), a syndrome with similar symptoms to niacin deficiency (Tahmoush et al,1976).