A 59-year-old man presented with a five-year history of an asymptomatic widespread truncal rash. Cutaneous biopsies showed mastocytosis, which was shown to be CD4-positive immunohistochemically. This potential diagnostic pitfall in the discrimination of mastocytosis from a variety of other haematopoietic neoplasms, particularly mycosis fungoides, is discussed.
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Competing interests: None.
Patient consent: Obtained.
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