RegIV (regenerating islet-derived protein 4) (also Reg-like protein, RELP, and gastrointestinal secretory protein) is a secreted glycoprotein belonging to the regenerating gene family of the calcium (C-type) dependent lectin superfamily (1, 2). There are three potential isoforms for human RegIV. Isoform 1 is synthesized as a 158 amino acid (aa) precursor with a 22 aa signal sequence and a 136 aa mature chain. Amino acids 30-156 constitute a C-type lectin-like domain, and aa 50 is a site of potential N-linked glycosylation. In addition, aa 127 and 142-143 form a ligand-binding surface. Isoform 2, shows a 79 aa substitution for aa 56-138, while isoform 3 shows an 11 aa substitution for the C-terminal 57 amino acids. In isoform 3, in a splicing variant at aa 102-112, the aa sequence is changed, and another splicing variant at 113-158 is missing. The mature RegIV, isoform 1, has a molecular weight of 17kD (3). Human RegIV shares 68% and 67% aa sequence identity with rat and mouse RegIV, respectively. Like other members of the regenerating gene family, RegIV is expressed in the gastrointestinal (GI) tract and ectopically at other sites in the setting of tissue injury (1, 2). In particular, RegIV is expressed in normal colon mucosa, and up-regulated in colon adenocarcinoma, pancreatic cancer, gastric adenocarcinoma, inflammatory bowel disease (Crohn’s disease and ulcerative colitis), and outside the GI tract in prostate adenocarcinoma (2-8). The physiological function of RegIV is presently unknown, but it may be involved in inflammatory and metaplastic responses of the GI epithelium (3).