Moles are abnormal tissues that appear during human development, and some have the potential to become cancerous. With the popularization of medical knowledge, people’s understanding of moles has become more and more comprehensive, but certain moles in specific areas are still easy to be ignored, the most typical of which is nail moles. The most common symptom of a nail nevus is a black nail line, which can easily turn into a melanoma. Nevi are most often seen in children and adolescents. Most nail nevi occur before the age of 20, but a few develop after the age of 20, mostly on the fingernails and a few on the toenails. The nail nevus differs from subxiphoid melanoma in that the latter lesions are often larger than 6 mm in diameter, asymmetrical in shape, with irregular and bleeding margins and uneven color. If any of these conditions occur, malignant melanoma should be highly suspected, but a hospital diagnosis is ultimately required. When the following symptoms occur in moles, it is necessary to check immediately whether malignant changes occur and melanoma is produced. 1.The lesion grows faster or the size of the area does not change significantly, but it is significantly thickened. 2. Color change, darkening, or especially the appearance of light blue tones. 3. The nevus extends radially to the surrounding area. 4.The nevus is painful or uncomfortable for no reason, and there is a small amount of exudate on the surface. 5.The lymph nodes in the area of the colored nevus are enlarged, and blue-black color is faintly visible. 6.Hair loss, scabbing or satellite foci appear. 7.The surface is broken, bleeding and ulcers are formed. 8.Unexplained symptoms such as itching or pain, inflammation appear. 9.Black urine. Three methods to prevent melanoma 1. For pigmented nevi that occur in easy to rub, biopsy should be taken for pathological examination. And remove all of them as early as possible to prevent the appearance of malignant melanoma. If a large hairy mole in children is in the waist, the main part can be removed as much as possible in the middle of the large hairy mole before malignant transformation, sutured on both sides, and then remove the rest after the surrounding skin of the melanoma patient is pulled loose, until all the moles are removed. The specimen of each excision must be sent for pathological examination. If there is malignant change, all of them should be removed and implantation should be performed. This is what needs to be noted in the prevention of malignant melanoma. 2. The prevention of malignant melanoma should not be stimulated by corrosive drugs or thorough freezing and other methods. Because the nevus of melanoma patients often stimulated by trauma can become malignant. 3, prevention of malignant melanoma should be done to pay attention to the information of malignant change of color moles. Patients with melanoma should be alert to information including the following: the volume of the nevus increases, and the pigment is either dark or light; the nevus expands radially to the surrounding area; the nevus is painful or uncomfortable for no reason, and there is a small amount of exudate on the surface; the lymph nodes in the area of the nevus are enlarged, and blue-black color is faintly visible. This is also a preventive measure for malignant melanoma.