which receptor subtype must form a dimer, once it binds to its ligand?
1 A ligand is any molecule that binds to a specific site on a protein or other molecule. In this case, the estrogen hormone 17-beta-estradiol is the ligand, and the estrogen receptor is the protein.  Binding turns on, or activates, a hormone receptor.
G-protein coupled receptors (GPCRs) [1]are the largest group of plasma membrane receptors of which rhodopsin and adrenergic receptors are the most familiar.