While primary CD, which is caused by mutations in the SLC22A5 gene encoding for organic cation/carnitine transporter 2 (OCTN2) [5], is a relatively rare disease with strongly reduced (≈90%) plasma and tissue carnitine concentrations, secondary CD displays a less strong decrease (≈30–50%) of tissue carnitine concentrations, but is more frequent in humans. This evidence concerns the gene SLC22A5 and Cowden disease.