The term "ectodomain" refers to the external portion of a transmembrane protein that protrudes from the cell surface. This region is often involved in ligand binding and receptor activation. Synonyms for ectodomain include extracellular domain, exterior region, outside portion, and outer zone. Other related terms might include cell surface domain, surface-exposed region, or plasma membrane component. Ectodomain regions are particularly important in signal transduction pathways, where extracellular signals are detected and transmitted across the cell membrane to initiate a cellular response. Understanding the molecular structure and function of ectodomains is key to developing new therapies for a wide range of diseases and conditions.