A 14-year-old boy with otherwise unremarkable acne for 2 years presented because the acne was much more severe in a lesion extending down his left arm (figure). This linear and atypically distal distribution, with a comedone in virtually every follicle of the affected area, suggests an unusual pathological basis. Disorders following this kind of linear or whorled pattern (lines of Blaschko) are thought to reflect the epidermal distribution of somatic mosaicism, whether due to X-inactivation in females carrying X-linked dominant mutations, or to mutation in embryonic precursors of the keratinocyte lineage. 1 Supporting this theory, somatic mutations of the keratin gene (K10) have been detected in naevoid epidermolytic hyperkeratosis. 2 In considering candidate genes for somatic mutation in our patient, we recalled the atypical generalised acne seen in Apert syndrome. 3 This complex congenital malformation, characterised by craniosynostosis and syndactyly, is due to specific germline mutations in the gene for fibroblast growth factor receptor 2 (FGFR2). 4 We examined this patient’s epidermal naevus for comparable somatic mutations in FGFR2. DNA was extracted from peripheral blood lymphocytes, from scrapings and keratinous plugs of lesional epidermis, and from banal follicular keratoses on the other arm. Scrapings routinely yielded 200–500 ng high-quality DNA. The region of FGFR2 containing the two major Apert syndrome mutations was amplified by PCR and digested with MboI and BglI, each of which has a single restriction site in the normal sequence abolished by the major Apert syndrome mutations 934C→ G and 937→ G, respectively. All samples showed normal digestion with BglI, but samples from lesional skin were partially resistant to MboI digestion whereas samples from the opposite arm and blood digested normally with MboI (figure). We cloned the PCR product and sequenced eight independent MboI-resistant clones. All contained the 934C→ G mutation (predicting a Ser252Trp substitution) identical to that in Apert syndrome. This was confirmed by blot hybridisation of the PCR product with a mutant oligonucleotide (figure). From two independent samples of the lesion, 56% and 34%, respectively, of cells were mutated (not shown).