First, 38% of identified genes in our study are known to harbor causative mutations for autosomal recessively inherited IEMs that often exhibit concordant but more extreme changes in the implicated metabolite, as exemplified by elevated urine levels of cystine in cystinuria (MIM 220100, SLC7A9) or tryptophan in Hartnup disease (MIM 234500, SLC6A19). The gene discussed is SLC7A9; the disease is Hartnup disease.