The function of SFXN5 (sideroflexin 5, ENSG00000144040) is as follows. Mitochondrial amino-acid transporter (By similarity). Transports citrate (By similarity). Does not act as a serine transporter: not able to mediate transport of serine into mitochondria (By similarity) (PubMed:30442778). In brown adipose tissue, plays a role in the regulation of UCP1-dependent thermogenesis probably by supporting mitochondrial glycerol-3-phosphate utilization (By similarity). {ECO:0000250|UniProtKB:Q8CFD0, ECO:0000250|UniProtKB:Q925N0, ECO:0000250|UniProtKB:Q9H9B4, ECO:0000269|PubMed:30442778}.