Pachyonychia congenita (PC) is a rare inherited condition that primarily affects the nails and skin. The fingernails and toenails may be thickened and abnormally shaped although 20/20 nails dystrophy is found only in one type of PC. The most debilitating feature is that affected individuals develop painful calluses and blisters on the soles of their feet (plantar keratoderma) and less frequently on the palms of their hands (palmoplantar keratoderma). Additional features include leukokeratosis (white patches on the tongue and inside of the mouth) sometimes misdiagnosed as thrush or leukoplakia; follicular hyperkeratosis (bumps around the elbows, knees, waistline or other friction points), and various cysts including numerous steatocystomas. Features may vary among affected individuals depending on their specific mutation. The classification of PC into two types has been replaced with a more correct system based on genetic testing identifying the specific keratin gene involved (see McLean, Journal of Investigative Dermatology, May 2011). The types are PC-K6a, PC-K6b, PC-K6c, PC-K16 and PC-K17. More than 100 specific mutations have been found which cause PC.