The P6-1 monoclonal antibody specifically recognizes the extracellular region of Human Epidermal Growth Factor Receptor 4 (HER4). HER4 is encoded by ERBB4 (erb-b2 receptor tyrosine kinase 4). HER4 (ErbB4) belongs to the epidermal growth factor receptor subfamily within the ERBB family of Receptor Tyrosine Kinases (RTK) that includes HER1 (EGFR), HER2 (ErbB2), and HER3 (ErbB3). HER4 (ErbB4) is an ~180 kDa type I transmembrane glycoprotein that contains an extracellular ligand-binding region, a transmembrane sequence, and a cytoplasmic tail with the active tyrosine kinase domain. This RTK functions in a variety of normal cellular processes including proliferation, differentiation, migration, survival, and apoptosis that are often dysregulated in cancer cells. HER4 (ErbB4) is prominently involved in regulating the development and maintenance of the heart, central nervous system, and mammary gland. Upon binding ligands, including neuregulins and EGF family members, HER4 (ErbB4) can homodimerize or heterodimerize with other family members and transmit intracellular signals involved in initiating and regulating cellular responses. High expression of ERBB4 occurs in brain, heart, kidney, and heart as well as skeletal muscle, bone marrow mesenchymal stem cells and breast tissue. Overexpression of and mutations in the ERBB4 gene have been associated with some leukemias, neuroblastomas, and carcinomas. The P6-1 antibody can reportedly suppress Neuregulin-1 (NRG-1)-dependent signaling and colony formation responses mediated through HER4 (ErbB4) expressed on human breast cancer cell lines.