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. The gene discussed is ZNF660; the disease is pachyonychia congenita.