Melanocytes produce the pigment, melanin, which generates our skin color.
They are located in the epidermis, the top layer of the skin. Usually melanocytes are independent units. However, collections of these cells can form, causing aberrant growth patterns. These growth patterns can cause the cells to grow and repopulate without stopping, forming a malignant tumor. These cells can then spread along the epidermis or invade deeper into the dermis, the bottom layer of the skin, and below.