The 4FR6D3 monoclonal antibody specifically recognizes CD334 which is also known as Fibroblast growth factor receptor 4 (FGFR4), Tyrosine kinase related to fibroblast growth factor receptor (TKF), or JTK2. CD334 is a ~110 kDa type I transmembrane glycoprotein that belongs to the fibroblast growth factor receptor (FGFR) in the Ig superfamily (IgSF). The ligand-binding extracellular domain has three immunoglobulin-like C2-type domains followed by a transmembrane region and a cytoplasmic tail which contains a tyrosine kinase domain. CD334 serves as a high affinity receptor that dimerizes upon binding to fibroblast growth factors including FGF-1, -2, -4, -6, -8, -9, -16, -17, -18, and -19. This cell surface receptor tyrosine kinase is widely expressed on epithelial cells, fibroblasts and skeletal muscle cells and plays a role in the proliferation, differentiation, migration, and survival of these different cell types. Abnormal expression and activity of CD334 has been associated with a number of different cancers.