Major facilitator superfamily (MFS), also called the uniporter-symporter-antiporter family, is a widespread transport system consist of single-polypeptide secondary carriers which capable only of transporting small solutes in response to chemiosmotic ion gradients. Typical MFS have more than 12 transmembrane segments and they combined with ABC superfamily regulate most uniport-symport-antiport process. MFS has diverse functions including transport of sugars, drugs and Krebs cycle metabolites and it is divided into 17 subfamilies depend on degrees of sequence similarity.