Since the extraction of plasma DNA with the same genetic changes as the primary tumor in 1989 and identification of mutated KRAS sequences in the plasma or serum from patients with PC in 1994, circulating tumor (ct) DNA is becoming a research hotspot with high potential as liquid biopsy marker in cancer medicine (Sorenson et al. 1994; Stroun et al. 1989). The gene discussed is KRAS; the disease is pachyonychia congenita.