Seventy-nine-year-old man with Langerhans cell histiocytosis treated with cladribine.

diagnosis or during follow-up. Our twins were unusual because of the onset of cutaneous lesions during adult life. One had systemic symptoms presumed to be related to the release of mast cellederived mediators (histamine, prostaglandins), but neither had evidence of extracutaneous disease. Age at onset influences the clinical behavior of mastocytosis. Pediatric mastocytosis often resolves spontaneously with time and is usually limited to the skin, with only about 10% of patients developing extracutaneous disease. Mastocytosis arising in adulthood (35% of cases) is often persistent and associated with involvement of other organs. It has been suggested that the divergent clinical behavior between pediatric and adult mastocytosis may be related to the presence or absence of genomic mutations, in particular involving the c-kit protooncogene. A significantly higher percentage of c-kit mutations (77%) was demonstrated in adult-onset mastocytosis compared with pediatric cases (42%). Therefore the absence of a mutation in our patients could possibly indicate a favorable prognosis comparable to that of childhood UP.