The spectrum of disease causing mutations is immense. It just so happens that the overwhelming majority of genetic alterations in the APC gene with leads to adenomatous polyposis coli generate truncated gene products. This observation lead to the development of the in vitro synthesis protein assay (protein truncation test) which is a sensitive method to detect these truncated gene products from patient samples. This article describes the assay to detect truncated proteins for the APC gene, which can also be applied to other disease causing genetic alterations which commonly lead to truncations such in HNPCC, von Hippel-Lindau, osteogenesis imperfecta, retinoblastoma, BCRAI, beta-thalassemia, hemophilia B, Duchenene and Becker muscular dystrophy.