In vitro synthesised protein assay; a procedure for detection of mutations that result in a protein of significantly different size, e.g. by early termination, insertion, change in reading frame or altered mRNA splice site. cDNA is prepared from RNA of a selected cell or tissue; PCR is performed using as a primer an oligonucleotide containing the promoter of the desired RNA polymerase and eukaryotic translation initiation sequence. The resulting amplicon can be translated so as to include a label (e.g. a radioactive amino acid or biotin-lysyl-tRNA) which will permit visualisation of the protein when separated by SDS/PAGE.