These are usually attributable to overactive sebaceous glands, which produce excess sebum. Retinoids reduce the amount of sebum produced by the sebaceous glands. Sebaceous carcinoma is a skin cancer of the sebum producing glands. The drug decreases the size and sebum output of the sebaceous glands. Meibomian glands in the eyelid secrete sebum to lubricate and protect the eye. "C. acnes" also provokes skin inflammation by altering the fatty composition of oily sebum. The main hormonal driver of oily sebum production in the skin is dihydrotestosterone. Pregnancy can increase androgen levels, and consequently, oily sebum synthesis. Excess keratin combined with sebum can plug the opening of the follicle. Androgens increase sebum (oil) production. Earwax is partly composed of sebum produced by glands in the ear canal. Carotenoids are eliminated via sweat, sebum, urine, and gastrointestinal secretions. Investigators believed the bacteria caused comedones, sebum production, and ultimately acne. Human skin also secretes sebum, which is toxic to most microorganisms. Another androgen effect is increased secretion of oil (sebum) from the skin.