Insulin-like growth factor-II (IGF-II) messenger RNA (mRNA)-binding protein-3 (IMP-3), also known as K homology domain-containing protein overexpressed in cancer (KOC) and L523S, is a member of the IGF-II mRNA-binding protein (IMP) family, which also includes IMP-1 and IMP-2. IMP-3 is a 580 amino-acid protein encoded by a 4350-bp mRNA transcript produced by a gene located on chromosome 7p11.5. As a translational activator of IGF-II leader-3 mRNA, it is associated with cell proliferation and is considered an oncofetal protein due to its expression during embryogenesis and in some malignancies. Normal adult tissues that express IMP-3 include term placenta, ovary, testis, brain, lymph node germinal centers, and intestinal mucosa. Increased levels of IMP-3 have been identified in pancreatic carcinoma, renal cell carcinoma, germ cell neoplasms, ovarian carcinoma, and extrapulmonary small-cell carcinoma, as well as high-grade neuroendocrine carcinoma, squamous cell carcinoma, and adenocarcinoma of the lung. Additionally, IMP-3 was shown to be a prognostic marker in patients with renal cell carcinoma, with lack of expression in the primary tumor predicting longer metastases-free survival.