Receptors, in general, are not highly expressed on endogenous cell surfaces. Adrenergic receptors (ARs) are commonly expressed in the femtomolar range. Correspondingly, their mRNA levels are also present in limited quantities. Northern analysis of the adrenergic receptors has confirmed this observation by requiring a high amount of poly A⁺ RNA (10–20 μg/lane) for the detection of the signal. To overcome this limitation, many investigators now use RNase Protection Assays, which are of higher sensitivity but also require much less poly A⁺ RNA for analysis. Thus, the lack of abundant mRNA makes receptor cloning studies difficult because of their underrepresentation in a given cDNA library.