A receptor is a protein molecule usually found inside or on the surface of a cell that receives a chemical signal from outside the cell. When such chemical signals called ligands bind to a receptor, they cause some form of cellular tissue response. For example, ligands include neurotransmitters, hormones, and growth factors. This binding initiates a chain of intracellular reactions, ultimately reaching the target cell nucleus and resulting in programmed changes in gene expression. Receptor status may influence diagnosis and treatment of certain diseases like Alzheimer’s disease and breast cancer.