<prompt> Answer this question truthfullyCan you explain what a sebaceous cyst is?  <response>: <reason> A sebaceous cyst a form of trichilemmal cyst is a closed sac or cyst below the surface of the skin that has a lining that resembles the uppermost part infundibulum of a hair follicle and fills with a fatty white, semi-solid material called sebum. Sebum is produced by sebaceous glands of the epidermis. 