Have you ever suddenly realized one day that you have an extra black line on your finger (toe) nail of your hand or foot? It doesn’t hurt or itch, and your first reaction is that you don’t care? Or are you worried that something bad has grown on your finger (toe) nails? In our clinical work, we often meet such patients, and many of them come to the hospital with doubts. This kind of black line is actually a nail mother mole, today we will come together to understand. First, what is a nevus? A nevus is a kind of nevus. The most common symptom of a nevus is a black line on the nail, which can easily turn into melanoma and is easily neglected. At present, the cause of nevus is unknown, and there are many factors affecting the formation of nevus of onychomycosis, including systemic pigmentation diseases, acanthosis nigricans, Addison’s disease, Peutz-Jeghers syndrome and vitamin B12 deficiency, Cushing’s syndrome after adrenalectomy, or Laugier-Hunziker syndrome. Sometimes trauma, fungal infections, eczema-like changes, subnail hemorrhage, certain chemotherapy drugs can also lead to the black line of the nail, but this black line of the nail is different from the mother-of-nail nevus. Symptoms As mentioned before, the most common symptom of nevus of onychomycosis is black line of nail, which is mostly seen in fingernail, thumb, little finger, and sometimes it can also involve toes, and the width of black line is usually less than 3mm, and the edge is regular and neat, and the width of the black line can be widened with the growth of adolescent patients, and it is usually not developing after adulthood, and usually the patients don’t have any discomforts. If the width of the black line is more than 6mm and the edge is irregular, the possibility of malignant transformation should be highly suspected. At present, the most reliable and precise treatment for nevus of nail is surgical excision. Remove the diseased nails and excise the nail bed, which can be directly sutured after local freeing and tension reduction. For patients with large lesions, the nail bed cannot be sutured directly, they can choose to have free nail bed transplantation, skin grafting or skin flap transfer to cover the trauma in the first stage or in the second stage after the return of pathologic results. The importance of pathological examination should be emphasized here, because nevus of nail is easy to become cancerous, pathological result is the only gold standard to determine the benign or malignant nature of the lesion. Prognosis: Generally the prognosis of nevus of onychomycosis is good, and some patients only have localized nail deformity and other complications. For malignant cases, further treatment should be carried out in strict accordance with the guidelines for melanoma treatment. Finger amputation and radiotherapy are needed if necessary, and regular follow-up observation is needed after surgery to prevent tumor recurrence and metastasis. In conclusion, don’t panic when black lines appear on your nails, and you can’t just leave it alone and let it develop, you still need to go to the hand surgery department of regular hospitals in time for treatment.