Current recommendations for APC mutation analysis in patients with familial adenomatous polyposis (FAP) advise full gene sequencing and gene dosage analysis replacing the protein truncation test (PTT) as a pre-screening tool. Here we report on two unrelated patients with classical FAP and unremarkable family history in whose leukocyte-derived DNA no pathogenic APC mutations could be identified. PTT analysis, however, evidenced aberrant bands in both patients with subsequent targeted mutation analysis of their tumour-derived DNA identifying two novel, pathogenic APC alterations present in a mosaic state, at blood levels (1 – 15%) below the detection limits of conventional Sanger sequencing. Our findings demonstrate the value of the PTT to identify mosaic mutations in apparently APC mutation-negative FAP patients with de novo classical polyposis and the need to keep the PTT within the diagnostic repertoire for APC mutation analysis. (By Karl Heinimann, http://jmg.bmj.com/content/early/2011/06/07/jmg.2011.089474.abstract?papetoc )