ERP44 is a 406 amino acid protein that contains one thioredoxin domain, a motif that participates in various redox reactions throughout the cell. Localized to the lumen of the endoplasmic reticulum (ER), TXNDC4 functions to inhibit the activity of IP3R-I (inositol 1,4,5-triphosphate receptor,type 1) within calcium channels. In addition, TXNDC4 is thought to regulate oxidative protein folding within the ER and may be involved in retaining proteins.