The availability of cloned cDNAs encoding the four subunits of the Torpedo acetylcholine receptor, which can be expressed to make functional receptors in Xenopus oocytes, has made possible a detailed investigation of the functions of the different structural components of the receptor. The functional analysis of receptors with alpha-subunits altered at specific sites by site-directed mutagenesis of the cDNA has allowed the location of specific regions of the alpha-subunit molecule involved in acetylcholine binding and forming a transmembrane ionic channel.