The protein truncation test (PTT) is a powerful mutation detection technique originally described for mutation screening of the dystrophin gene (1 ). It is based on in vitro transcription/translation technology, and is capable of directly scanning kilobase-sized fragments of DNA for chain-terminating mutations. PTT should be the mutation detection technique of choice to screen genes in which the majority (if not all) of the disease-causing mutations result in the formation of a truncated protein product.