Malignant melanomas are most commonly found in the skin and are generally the same as nevi but with some differences. There is only one malignant cell type that makes up all malignant melanomas, the malignant pigment-producing cells called melanocytes. Malignant melanomas are most commonly found in the skin and are generally the same as nevi but with some differences. There is only one malignant cell type that constitutes all malignant melanomas, the malignant pigment-producing cells called melanocytes, but they are somewhat rarely distinguished by their morphology, such as spindle-shaped and cylindrical. 1. Asymmetry: The two halves of a normal mole are symmetrical, while the two halves of a malignant melanoma are asymmetrical. 2. Border: The edge of common nevus is smooth and clearly demarcated from the surrounding skin, while the edge of malignant melanoma is uneven and jagged. In addition, the surface is rough with scaly or flaky flaking, and sometimes there is oozing or blood oozing, and the lesion is higher than the skin. 3. Color: Common moles are usually tan, brown or black, while malignant melanoma will be mixed with pink, white and blue-black on top of tan or brown. Among them, blue is the most ominous, and white suggests that the tumor has degenerated on its own. Nodular malignant melanoma is always blue-black or gray in color. 4. Diameter (diameter): The diameter of common mole is usually less than 5 mm, while the diameter of malignant melanoma is greater than 5 mm. Benign melanin plaques are nevi, of which junctional nevi are most likely to become malignant, mixed nevi are less likely, and endothelial nevi are rarely malignant. However, most of the scalp melanomas are not transformed from nevi, so it is thought that the disease is not entirely related to nevi. In addition to distinguishing melanoma from nevus by appearance, special attention should be paid to nevi that itch and are painful to press. These nevi are likely to develop ulcerative lesions if they continue to develop, and the stinging or burning pain will be more pronounced, and satellite nodules or enlarged regional lymph nodes will appear around them, indicating that the disease has progressed to an advanced stage. Malignant melanoma can appear in rare areas such as under the fingernails of the fingers or toes, or on the mucous membranes of the mouth, vagina, or inside the anus. Primary malignant melanoma is also found on the pigmented tissue covering the brain (meninges). It is important to emphasize that the distinction between poorly structured nevi and early malignant melanoma is difficult to make by visual observation alone, and biopsies should be performed promptly to obtain a pathological confirmation of the suspected lesion.