Human metabotropic glutamate receptor 5 (mGluR5; also mGluR5b) is a 150 kDa, 7-transmembrane glycoprotein that belongs to group I of the C-family of G-protein coupled receptors. mGluR5 is constitutively expressed and regulates neuronal ion channel activity. Human mGluR5 is 1212 amino acids (aa) in length and contains an extracellular domain (ECD) of 558 amino acids. Through its ECD, mGluR5 either homodimerizes or heterodimerizes with the Ca2+-sensor receptor. There is one alternate splice form (mGluR5a) that shows a 32 aa deletion between aa 877 - 908 in the cytoplasmic tail. Over aa 19 - 509, human mGluR5 is 98% aa identical to mouse, rat, and canine mGluR5.