Interestingly, some of these genes, i.e. KRT1, KRT9, KRT16, DSG1, DSC2 and JUP, are mutated in hereditary PPKs [16, 17], while the ABCA12 gene is defective in harlequin ichthyosis characterized by the loss of the skin lipid barrier [46]. Here, KRT9 is linked to Harlequin ichthyosis.