MMP9 and pachyonychia congenita: Based on the previous studies on PC subtyping [10], we selected Periostin (POSTN), Fibronectin 1 (FN1), Secreted protein acidic and cysteine rich (SPARC), Matrix metalloproteinase 9 (MMP9) and Thrombospondin 2 (THBS2) as the markers of activated stroma as they are exclusively expressed by PC stroma with no contribution from the epithelial compartment, and also correlate with the poor prognosis in PC (Figure 4).