Further evaluation of biological function, tissue specificity, as well as assessment of the overlap compared to previous investigations of serum and seminal plasma, resulted in the selection of four proteins [Follistatin, Chemokine (C-X-C motif) ligand 16, Pentraxin 3 and Spondin 2 (SPON2)] to be validated in 42 serum samples from patients with or without PC [75]. This evidence concerns the gene SPON2 and pachyonychia congenita.