Early onset weakness in the upper limbs and neck is almost invariably seen in patients with mutations in SLC52A2, in contrast to those patients with SLC52A3 mutations or genetically unclassified Brown-Vialetto-Van Laere syndrome, in whom the onset of weakness is often more generalized (Green et al., 2010; Bosch et al., 2011). Here, SLC52A2 is linked to riboflavin transporter deficiency.