In general, it has been difficult to obtain antibodies which are useful for immunoblotting of endogenous seven-transmembrane receptors (7TMRs) despite the claims made by many companies on commercially available antibodies. In this review, we will use the mu opioid receptor (MOPR) in brain as an example to underscore the importance of using knock-out (K/O) mice and multiple independent approaches (ligand affinity-labeling, receptor phosphorylation and immunoblotting) in identifying 7TMRs following sodium dodecyl sulfate - polyacrylamide gel electrophoresis (SDS-PAGE). The rigor and convergence of pharmacological and biochemical data provide confidence on the unequivocal identification of MOPR. The distinct relative molecular masses (Mr's) and band patterns are largely due to variations in the extent of N-glycosylation in different cell lines, brain regions and species.