Black nevus, generally referred to as pigmented nevus, appears mainly due to hereditary factors, and to a lesser extent, it may also be affected by ultraviolet irradiation, stimulation by harmful substances, immunosuppression and ionizing radiation. 1. Genetic factors. A benign tumor formed when melanocytes gather under the skin due to genetic factors. 2. Ultraviolet radiation. Prolonged exposure to ultraviolet rays can lead to melanin deposition, which may aggravate or induce the formation of nevus. 3. Stimulation of harmful substances. Prolonged stimulation of the skin by irritating chemicals can trigger the formation of nevus, the more common harmful substances, including paint, formaldehyde and hair dyes and so on. 4. Immunosuppression. Under the influence of immunosuppression, melanocytic nevus may be induced. 5. Ionizing radiation. Prolonged exposure to ionizing radiation, such as X-rays, is also an important cause of nevus formation. There are more factors that can affect nevus in clinic, and specific cases need to be analyzed.