Thioredoxin is a class of small redox proteins known to be present in all organisms. It plays a role in many important biological processes, including redox signaling. The thioredoxin (Trx) fusion E. coli expression system is available to offer soluble expression of normally insoluble or difficult to express proteins. It was reported that a number of mammalian cytokines and growth factors, when expressed as C-terminal trxA fusion proteins, stayed remarkably soluble in the E. coli cytoplasm under certain conditions.