The adrenergic receptor family has three main members: alpha-1, alpha-2, and beta receptor types. Each of these three types contains three or more subtypes. There are currently four pharmacological (2A, 2B, 2C, and 2D) and three molecular (or genetic) alpha-2 adrenergic subtypes (2A/D, 2B, and 2C). Because different functions are likely to be mediated by different subtypes, much effort is being directed towards understanding the physiological roles of the various subtypes. However, little is currently known about the specific function mediated by the various subtypes.