A targeting vector was designed to insert a sequence containing the N-terminal 133 amino acids of Chrng fused to 382 amino acids of Chrne, including transmembrane segments 1-4. Mutants lacked endogenous transcripts at birth, however, mutant transcript levels matched levels of other cholinergic receptor subunits. Mutant transcript levels then decreased during postnatal development, as expected for regulatory sequences in the endogenous locus.