The differentiation process in which basal epidermal cells gradually mature and transform into stratum corneum cells is known as keratinization. During keratinization, a tough, chemically resistant, cross-linked protein band is laid down just inside the plasma membrane, and the whole cell flattens to a thin disc, or corneocyte. Histopathology can be used for diagnosis in some settings, particularly to improve diagnosis in genetic ichthyoses. There has been some discussion of topical tacrolimus being helpful in the early lesions, while systemic steroids may be used for neurologic disease. The presence of epidermal nevi can be an isolated finding or merely a marker of an underlying syndrome, of which there are many epidermal nevi syndromes. There are many congenital disorders that present with epidermal nevi in conjunction with various systemic manifestations, and multiple genes have been implicated, including FGFR3, HRAS, PIK3CA, PTEN, and AKT1, among others.