Epidermal naevi occur when early in the life of the embryo a genetic mutation develops in one cell, which divides and populates the cells along a line of development (Blaschko’s lines). This mutation is not found in other cells of the body.
To date mutations have been found in HRAS, KRAS, PIKC3A, FGFR (fibroblast growth factor 2 and 3), KRT1 and KRT10 (keratin) when the skin is sampled. However, skin biopsy is not the usual way for making the diagnosis.