SLC6A4 and Friedreich ataxia: Other SLC6 gene family members upregulated in the FRDA samples include those encoding a Na+/Cl−-dependent serotonin transporter (SLC6A4) localized in the plasma membranes of cells in CNS tissues as well as in neurons of the peripheral nervous system (Ramamoorthy et al., 1993), and a Na+-dependent branched-chain-amino-acid transporter (SLC6A15) predominantly expressed in the brain (Takanaga et al., 2005).