SLC22A5 and systemic primary carnitine deficiency disease: Primary carnitine deficiency (PCD, OMIM #212140) is an autosomal recessive carnitine transport defect caused by biallelic pathogenic variants in the solute carrier family 22 member 5 (SLC22A5) gene, which encodes organic cation/carnitine transporter type 2 (OCTN2) (Nezu et al., 1999; Tang et al., 1999; Picard, 2022).