Fibroblast growth factor receptor 4 (FGF R4), also known as CD334, is a 110 kDa glycosylated transmembrane receptor tyrosine kinase (1). Mature human FGF R4 consists of a 348 amino acid (aa) extracellular domain (ECD) with three Ig‑like domains, a 21 aa transmembrane segment, and a 412 aa cytoplasmic domain that contains the tyrosine kinase domain (2). Within the ECD, human FGF R4 shares 90% and 88% aa sequence identity with mouse and rat FGF R4, respectively. Alternate splicing generates a potentially secreted isoform with a substitution encompassing the transmembrane segment (3). A 65 kDa N‑terminally truncated isoform lacks the signal peptide and first two Ig‑like domains. This isoform is produced in human pituitary adenomas and is constitutively phosphorylated and oncogenic (4). FGF R4 is widely expressed during embryonic development and in adult liver, kidney, and lung (5‑8). It binds FGF acidic, FGF basic, FGF‑8, -15, and -19 (2, 7, 9‑12). FGF R4 associates with beta-Klotho and sulfated glycosaminoglycans, and these interactions increase the affinity of FGF R4 for its ligands as well as its signaling capacity (8, 9, 12). FGF-19 induced signaling through FGF R4 is important for the regulation of bile acid synthesis and lipid and glucose homeostasis (10, 13). FGF R4 supports glucose tolerance and insulin sensitivity and protects against hyperlipidemia (13). It is down‑regulated in the liver during fasting and is up‑regulated by insulin (10). It can exert either proliferative or apoptotic effects on hepatocytes (8, 11). FGF R4 signaling is additionally required for skeletal muscle development in limbs (7, 14). FGF R4 interacts in cis with cell surface MMP-14, leading to down‑regulation of both proteins (15). In contrast, the Arg388 variant of FGF R4, which is associated with tumor progression in human cancer, is activated and stabilized by MMP-14 (15, 16).