A Becker naevus (Becker nevus) is a late-onset epidermal naevus or birthmark occurring mostly in males. It is also known as Becker melanosis. It is due to an overgrowth of the epidermis (upper layers of the skin), pigment cells (melanocytes) and hair follicles. It develops during childhood or adolescence on the shoulders or upper trunk, occasionally elsewhere.
It is thought that it is due to a gene defect, which has not yet been identified. It may be triggered to develop by circulating androgens (male hormones such as testosterone), which is why it appears in males at puberty.
A Becker naevus is a large one-sided brown patch, sometimes over half the upper back or chest. After puberty it often becomes darker and quite hairy, a feature also called “hypertrichosis”. Occasionally acne may develop in the naevus.
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