We are interested in control of epidermal gene expression by the AP1 family of transcriptional regulators.23 This family includes c-jun, junB, junD, c-fos, Fra-1 and Fra-2.1 We recently showed that inactivation of suprabasal AP1 factor function in epidermis causes a phenotype characterized by hyperplasia, hyperkeratosis, parakeratosis and pseudoainhum that resembles human keratoderma.5 The present study expands on these findings and examines the impact of AP1 factor inactivation on the structure and composition of the CE, and on epidermal keratinocyte gene expression. This evidence concerns the gene FOS and Hyperkeratosis.