Epidermal growth factor (EGF) is a growth factor and the founding member of the EGF family. All EGF family members are synthesized as type I transmembrane precursor proteins that may contain several EGF domains in the extracellular region. The mature proteins are released from the cell surface by regulated proteolysis. EGF is present in various body fluids, including blood, milk, urine, saliva, seminal fluid, pancreatic juice, cerebrospinal fluid, and amniotic fluid. Four ErbB (HER) family receptor tyrosine kinases including EGFR/ErbB1, ErbB2, ErbB3 and ErbB4, mediate responses to EGF family members. These receptors undergo a complex pattern of ligand induced homo or heterodimerization to transduce EGF family signals. EGF binds to the receptor EGFR stimulating the intrinsic protein-tyrosine kinase activity of the receptor. The tyrosine kinase activity initiates a signal transduction cascade that results in a variety of biochemical changes within the cell, including a rise in intracellular calcium levels, increased glycolysis and protein synthesis, and increases in the expression of certain genes including the gene for EGFR, which lead to DNA synthesis, cell growth, proliferation and differentiation. Other biological activities ascribed to EGF include epithelial development, angiogenesis, inhibition of gastric acid secretion, fibroblast proliferation, and colony formation of epidermal cells in culture. Defects in EGF are the cause of hypomagnesemia type 4 (HOMG4), also known as renal hypomagnesemia normocalciuric. HOMG4 is a disorder characterized by massive renal hypomagnesemia and normal levels of serum calcium and calcium excretion. Clinical features include seizures, mild-to mederate psychomotor retardation, and brisk tendon reflexes.