According to statistics, cancerous nevus occurs in 1 out of every 100,000 people. Although this ratio is small, melanoma resulting from cancerous nevus is a tumor with high malignancy, early metastasis and high risk, which can generally kill the patient within 1-2 years if not treated in time. Everyone has moles, but which moles are prone to cancer? There are four kinds of nevi: 1. Junctional nevus: located at the junction of epidermis and dermis. They are mostly found on the palms of hands, soles of feet, lips of mouth and external genital area. The surface is flat or slightly high, and the size is between 1-2 mm, light brown, brown-black or blue-black. There is a possibility of cancer, which can occur as melanoma. 2. Intradermal nevus: exists in the dermis. The surface is smooth and the boundary is clear. It is larger than 1 mm, grows in sheets, and is flat or slightly elevated. The color is dark and uniform, light brown, dark brown or inky black. Usually no cancerous transformation occurs. 3.Mixed nevus: It is a mixture of the above two kinds of nevi, generally like an intradermal nevus, and can also become cancerous because it has the components of junctional nevus. 4.Blue nevus: It is called blue nevus because of the presence of collagen beam light reflection on the surface of pigment cells. It is less common and occurs mostly in childhood. They occur on the back of the arms, hands and feet. Mostly benign, but occasionally cancerous. Although moles can become cancerous, the incidence is very low and we cannot remove all of them. In order to prevent the problem before it happens, experts suggest that people who are over 40 years old should perform self-examination once a month: take off the whole body clothes, hold a small mirror in front of the whole body mirror, and check the moles on the whole body skin, especially if there are any newly found moles. Melanoma is characterized by: untidy edges, irregular meandering and twisting; not clear black, but miscellaneous colors, interlocking; it is usually impossible to divide it into two symmetrical parts with a straight line; it is small at first and not easily detectable, but grows progressively, and is immediately visible when it grows to the size of an eraser head on a pencil. A normal nevus, on the other hand, has neat edges, is uniformly black or dark brown, and can easily be divided into two symmetrical parts with a straight line. Melanoma, if detected early, can be completely removed under local anesthesia with no worries.