The level of hypermethylated ctDNA detected in serum samples from patients with PC correlated positively with pathological T‐stage for ST6GALNAC3 and HAPLN3 (test for trend, P < 0.05; Fig. 3B) and was borderline significant for ZNF660 (P = 0.08) but nonsignificant for CCDC181 (P = 0.235) (Fig. 3B). This evidence concerns the gene CCDC181 and pachyonychia congenita.