SMAD4 and pachyonychia congenita: Tissue biomarkers in PC might be replaced by blood biomarkers.CA19–9 is the sole serum biomarker authorized by worldwidestandards to track PC response.23 PC ischaracterized by genetic and epigenetic changes specific to tumors,including common mutations in the four driver genes: TP53, KRAS, CDKN2Aand SMAD4.