GSTP1 and pachyonychia congenita: In the present study, we tested a panel of ten genes (SSBP2, MCAM, ERα, ERβ, APC, CCND2, MGMT, GSTP1, p16 and RARβ2), previously found to be methylated in PC [12, 13, 20–26], however, most of them have not been previously tested in serum from PC patients to determine the potential of these genes as non-invasive biomarkers for PC.