Among the different families of transporter, only two occur ubiquitously in all classifications of organisms. These are the ATP-Binding Cassette (ABC) superfamily and the Major Facilitator Superfamily (MFS). The MFS transporters are single-polypeptide secondary carriers capable only of transporting small solutes in response to chemiosmotic ion gradients [<cite idref="PUB00007278"/>, <cite idref="PUB00007279"/>].This entry represents the nucleoside efflux permease-inosine (NepI) subfamily of the MSF superfamily. These proteins are nucleoside efflux pumps and are involved in the efflux of purine ribonucleosides, such as guanosine, adenosine and especially inosine [<cite idref="PUB00054939"/>, <cite idref="PUB00054940"/>]. Purine ribonucleoside efflux pump, NepI