A sebaceous cyst is a benign, closed, firm sac that lies in the skin and often forms a dome-shaped lump attached to the epidermis that can be moved over underlying tissues. It occurs primarily on the face, neck, shoulder, or chest (hair-bearing areas of the body). It is extremely common in the population and may occur at any age. They are not contagious and do not run the risk of developing cancer (in other words, they're benign). However, they can get infected and be a bit unsightly.