This family contains sideroflexin 1, 2 and 3 (SFXN1/2/3). SFXN1 is a multipass mitochondrial membrane protein and is part of the sideroflexin family of proteins conserved throughout eukaryotes. In humans, SFXN1 is highly expressed in the blood, liver, and kidney, which are tissues with high one-carbon metabolism activity. SFXN1 functions as a mitochondrial serine transporter in one-carbon metabolism. SFXN1 and SFXN3 (and perhaps SFXN2) are the main mitochondrial serine transporters in human cells and Sfxn1-3 also have this function in D. melanogaster. SFXN1 and SFXN3 likely have other physiologically relevant substrates besides serine, such as alanine or cysteine. Because SFXN1 is expressed in many cancers most highly in leukemias and lymphomas and its expression is likely regulated by the Myc transcription factor, it is suggested that it may have roles in cancer cell growth. Furthermore, Myc-binding sites were found in the promoters of SFXN1, SFXN2, and SFXN3 [1].

1: SFXN1 is a mitochondrial serine transporter required for one-carbon metabolism. Kory N, Wyant GA, Prakash G, Uit de Bos J, Bottanelli F, Pacold ME, Chan SH, Lewis CA, Wang T, Keys HR, Guo YE, Sabatini DM; Science. 2018; [Epub ahead of print] PMID:30442778