SLC6A19 and Hartnup disease: Our findings indicate that 9 out of the 18 analysed B0AT1 mutations associated with Hartnup disease (R57C, G93R, R95P, R178Q, L242P, G284R, S303L, D517G, and P579L) lead to substantial retention of the mutated B0AT1 transporter within the endoplasmic reticulum (ER), suggesting impaired trafficking to the plasma membrane.