The function of Ensembl gene identifier ENSG00000163393 (SLC22A15, solute carrier family 22 member 15) is as follows. Organic zwitterion/cation transporter with apparent specificity for amino acids and their derivatives. Has low affinity for its substrates and may regulate their flux across the plasma membrane at high substrate concentrations (PubMed:33124720). Bidirectionally transports carnitine and acetylcarnitine, possibly regulating their cytosolic abundance and further fatty acid catabolism via beta oxidation (PubMed:33124720). Displays high transport activity toward zwitterionic substrates such as glycine betaine and diet-derived ergothioneine and carnosine. Can transport cations having an indole skeleton such as thiamine with lower efficiency. Does not transport agmatine (PubMed:15028572, PubMed:33124720). The transport mechanism, symport with sodium or facilitated diffusion allosterically regulated by sodium, remains to be elucidated (Probable). {ECO:0000269|PubMed:15028572, ECO:0000269|PubMed:33124720, ECO:0000305|PubMed:33124720}.