Recently, some patients always ask about the source and prognosis of nevus. Looking at the anxious patients, I compiled the following information for reference. First of all, we need to understand what a nevus is called. A nevus is a benign skin tumor caused by the proliferation of nevus cells. The common clinical manifestation of nevus is the black line under the nail. Common causes include systemic pigmentary diseases, pigmentation, vitamin deficiency, and can also be related to trauma, irritation, viral infection, and low immune function of the body. However, clinically it is often discovered unintentionally by patients or family members. Generally, no special treatment is necessary. Congenital nevus cell nevus, with the possibility of melanoma, is generally better to be surgically removed, and histopathological examination is needed at the same time, and nail extraction is not advisable to miss the diagnosis and cover up the developmental changes of the original condition. If found in young children, there is no need to rush to surgery, just pay attention to observation. If ① sudden increase in size ② darkening of color ③ surface erosion, exudation, bleeding, ulceration, swelling ④ self-conscious pain or itching ⑤ satellite lesions around the nail should be removed immediately and histopathology should be done. If a patient over 40 years of age develops a black line under the nail and changes rapidly in a short period of time, special attention should be paid. It may belong to acral lentiginous melanoma ALM, which is a common type of malignant melanoma in China. Acral lentiginous melanoma ALM mainly occurs in the palmoplantar, nail and perineal areas, and appears as unevenly pigmented patches with irregular borders. If located in the nail matrix, the nail plate and nail bed may show longitudinal bands of pigmented streaks. This type progresses rapidly, often enlarges within a short period of time, ulcers and metastases occur, and its five-year survival rate is low, only 11%-15%. The histopathology laboratory is the main basis for the diagnosis of malignant melanoma. The most valuable indicator for determining prognosis is the depth of tumor infiltration, so the most practical staging for diagnosis, treatment and statistics is simply to divide it into melanoma in situ and invasive melanoma, and then to stage it according to TNM. The Clark grading method is used to indicate the depth: Grade I melanoma cells are confined to above the epidermal basement membrane. Grade II invade the papillary layer of the dermis Grade III invade the vascular plexus under the papillary layer of the dermis Grade IV invade the reticular layer of the dermis Grade V invade the subcutaneous fat layer. The thickness is indicated by Breslow method, which is a visual micrometer to measure the thickness of the tumor. Histopathological diagnosis of melanoma is mainly based on structural form and tumor cell morphology. The morphology of nevus cells is diverse and can be polygonal, small round, spindle-shaped, vacuolated, dendritic, or oddly-shaped cells. They may be sweat-pigmented or non-pigmented. The nuclei and nucleoli are often large, with irregular nuclei and a nuclear division phase. The cellular heterotypic advantage of in situ melanoma is not obvious and needs to be analyzed in close clinical context in addition to relying on the structural form. In situ melanoma: ① relatively large, larger than 6mm ② tumor is asymmetric ③ melanocyte nests are of different sizes, irregular in shape and tend to fuse ④ melanocytes are scattered in all layers of the epidermis. ⑤Melanocyte horizontal expansion, unclear boundary ⑥Melanocyte atypical ⑦Cell necrosis ⑧Tumor cells are breakthrough epidermal basement membrane. Invasive melanoma: ① often have the intra-epidermal features of in situ melanoma ② tumor cells in the dermis are often distributed in a nest-like pattern, surrounded by reticulated fibers around the nest ③ cells at the base of the tumor are still in a nest-like pattern, with large cells and containing pigment ④ tumor cells in the lymphatic vessels or within the blood vessels ⑤ small blood vessels in and around the tumor are hyperplastic ⑥ lymphocyte infiltration, and plasma cells may be present. Histological indications of suspected malignant transformation are: ① single or clusters of melanocytes appear in the upper epidermis ② nevus cells at the true epidermal junction show atypical hyperplasia, arranged in nests, and may be irregularly distributed in the basal cell layer ③ usually nevus cells in the dermis gradually become smaller and longer from superficial to deeper layers, becoming small spindle-shaped cells, and the cells in the deeper dermis do not become smaller in malignant transformation ④ malignant transformation is mainly characterized by deep nuclear staining, enlargement (4) The characteristics of malignant changes are mainly deep nuclear staining, enlargement, irregular morphology, and atypical nuclear division phase.