Finally, as proof of principle, we investigated whether hypermethylated ctDNA for ST6GALNAC3 and ZNF660, as well as for two of our previously identified top candidate hypermethylated genes in PC tissue (CCDC181 and HAPLN3 (Haldrup et al., 2013)), could be detected in serum samples from patients with PC. This evidence concerns the gene HAPLN3 and pachyonychia congenita.