Becker's nevus is a non-cancerous, large, brown birthmark occurring mostly in males. It can be present at birth, but is usually first noticed around puberty. It typically occurs on one shoulder and upper trunk but occasionally occurs elsewhere on the body. A Becker's nevus often becomes darker, and excessive hair growth (hypertrichosis) and acne may develop within the nevus.
Becker's nevus is due to overgrowth of the epidermis (upper layer of the skin), pigment cells (melanocytes) and hair follicles. The specific underlying cause is unknown.
Because it often forms around puberty in males and is sometimes associated with acne and hair growth, its development may be triggered by androgens (male sex hormones such as testosterone).