Variations in the KRT6A, KRT6B, KRT16 and KRT17 genes are, in large part, associated with a rare disease related to thickening and abnormal shaped of fingernail and toenail, pachyonychia congenita (PC) (Table 1); while an absence of the Krt16 gene in mouse causes thickening skin of palms and feet, palmoplantar keratoderma (PPK) [104]. Here, KRT6A is linked to pachyonychia congenita.