SLC52A3 and riboflavin transporter deficiency: Thus far, three separate subtypes of RTD2 are described—type 1, 2 and 3—but, previously, RTD was classified as two separate genetic defects: Brown–Vialetto–Van Laere syndrome and Fazio–Londe syndrome, caused by mutations in the SLC52A2 and SLC52A3 genes, respectively.