Table I summarizes the properties of all of the dopamine receptors that have been cloned to date. Thus far, five different genes encoding pharmacologically distinct receptors have been identified and isolated. Based on their structural, pharmacological, and functional similarities, two of these, D1A and D1B (or D1 and D5), comprise the D1 subfamily. D2, D3, and D4 receptors represent a D2 subfamily whose members are also structurally and pharmacologically similar. In fact, given these considerations, it has been suggested that the D2, D3, and D4 receptors be termed the D2A, D2B, and D2C receptors, respectively, in recognition of their D2-like properties. Given the unexpected heterogeneity of the dopaminergic receptor system, it is logical to ask if there are other dopamine receptor subtypes remaining be identified. This seems probable, as the characteristics of the cloned subtypes do not match all of the properties of some dopamine receptors which have been previously investigated. For instance, there is extensive evidence that "D1-like" dopamine receptors exist which are linked to the activation of phospholipase C, phosphatidylinositol turnover, and Ca2+ mobilization. Dopamine, as well as several "D1-selective" agonists, has been shown to stimulate phosphatidylinositol turnover in both brain slices and kidney membranes (Felder et al., 1989; Undie and Friedman, 1990; Vyas et al., 1992), and injection of striatal mRNA into Xenopus oocytes leads to dopamine-stimulated phosphatidylinositol turnover and Ca2+ mobilization (Mahan et al., 1990). These dopamine receptors might be analogous to the alpha 1-adrenergic receptors which stimulate phospholipase C activity and might define a third distinct subfamily of dopamine receptors. There is also evidence for additional members of the D2 subfamily of receptors. Using gene transfer methods, a receptor with D2-like pharmacology has been identified and expressed but not yet sequenced (Todd et al., 1989). Also, a D2-related receptor has been characterized in kidney inner medulla membranes (Huo et al., 1991). It thus appears that there may be more dopamine receptor subtypes yet to be discovered.