The penetrance with respect to skin signs known to be associated with FLG haploinsufficiency15–17 (summing ichthyosis, keratosis pilaris, palmar hyperlinearity and flexural eczema) is 100% in homozygotes, 87·8% in heterozygotes and 46·5% in wild-type homozygotes (P< 0·0001, Fisher exact test). The gene discussed is FLG; the disease is keratosis pilaris.