Importantly, normal HVA:5-HIAA ratios of 1.6–3.4 (normal range 1.0–4.0) (Ng et al. 2015) in all POLG patients discriminate these profiles from other primary neurotransmitter disorders, such as dopamine transporter deficiency syndrome (DTDS), where the HVA:5-HIAA ratios are commonly above 5 (Ng et al. 2015). This evidence concerns the gene POLG and SLC6A3-related dopamine transporter deficiency syndrome.