Genetics is thought to be the primary cause of acne in 80% of cases.[2] The role of diet and cigarette smoking is unclear, and neither cleanliness nor exposure to sunlight appear to play a part.[2][13][14] In both sexes, hormones called androgens appear to be part of the underlying mechanism, by causing increased production of sebum.[5] Another frequent factor is excessive growth of the bacterium Cutibacterium acnes, which is normally present on the skin.[5]