Values are the number of amino acids in the predicted receptor protein. Percentages indicate the degree of sequence similarity between receptor types. References: Arai et al., 1990;Sakuri et al., 1990; Hosoda et al., 1991;Kozuka et al., 1991; Lin et al., 1991;Nakamuta et al., 1991; Elshourbagy et al., 1992; Hosoda et al., 1994; Nishimura et al., 1995; Yang et al., 1998.