The nicotinic acetylcholine receptor (nAChR) is a complex molecule consisting of four different subunits α, β, γ and δ which form a pentameric structure containing two copies of the α-chain and one copy of the others (reviewed in 1). The α-chain, which is the most highly conserved across species, is known to carry the binding site for cholinergic ligands (reviewed in 1) and the main immunogenic region (MIR), against which most antibodies induced by immunisation with native nAChR and most naturally-occurring autoantibodies are directed (reviewed in 2). The nAChR from Torpedo electric organs can be obtained in large quantities but that from the muscle or nervous system of other species can be purified only in small amounts. Similar limitations exist for many other receptors. Given the availability of sequence data on the nAChR (see this volume), we have used this as a model system to ask the following questions: 1. Using expression cloning of cDNA restriction fragments, can individual functional domains be studied independently? 2. Can we readily produce large amounts of otherwise rare proteins for structural studies? 3. Can cDNA libraries be successfully screened using radio-labelled ligand?