Everyone has a mole, which is formed by the abnormal increase and accumulation of melanocytes or melanin particles secreted by melanocytes in the skin. For example, common moles are formed by the accumulation of increased melanocytes into nests or balls; However, in Ota nevus and Freckle nevus, more melanocytes are scattered, which can show different colors because of their different distribution depth.

There are many kinds of nevus, the most common ones are localized nevus (which can be divided into borderline nevus, intradermal nevus and mixed mole according to the depth), in addition, there are giant nevus (congenital melanocytic nevi with a huge area), halo nevus (melanocytic nevi with hypopigmentation's surrounding skin), simple freckle nevus (mole) and freckles (which appear in exposed parts such as face and are brown. The color deepens after sun exposure), coffee and milk spots (light brown spots appearing at birth), chloasma (light brown spots appearing on cheeks and forehead of healthy women), Ota nevus (bluish brown nevus on eyes and maxilla), Ito nevus (bluish brown nevus on trunk and limbs), and blue nevus (blue macula or nodule).