Nevus is a benign proliferation of melanocytes, and its formation has many causes, which may be related to ultraviolet radiation, trauma, hormone level change, gene mutation and other reasons, and the cause of some nevi is not known. According to the time of nevus formation, it can be divided into two categories: congenital and acquired, in which the cause of congenital nevus has not been studied thoroughly, or it may be related to the gene mutation of the embryo in the process of development, which leads to benign proliferation of melanocytes. Acquired nevus is formed due to prolonged exposure to ultraviolet rays. In addition, trauma to the skin, indiscriminate use of medication, and changes in hormone levels in the body may induce nevus. Most of the moles do not cause abnormal sensation to the patients, only a few moles may cause itching, pain or sudden bleeding in different degrees. Moles have a certain chance of malignant changes, once you find that the shape or color of a mole has changed, or it grows too fast, or it breaks down and bleeds, you should go to a regular hospital for diagnosis and treatment in a timely manner.