A mouse eye is a pigmented nevus that accompanies us from birth. It is a diffuse patch of irregular shape that is formed by excessive cell growth. It is usually brown in colour and is completely covered by hair. It often appears on the face, neckline, arms, legs or under the chin. The nevus of the mouse should be monitored by a dermatologist, as exposure to sunlight can lead to the nevus turning into a malignant skin cancer, namely melanoma.