Humulone is one of five alpha acid analogues in hop resin, the others being adhumulone, cohumulone, prehumulone, and posthumulone. These analogues differ only slightly from each other in their molecular structures, and together they serve as precursors to iso-alpha acids, which are the predominant contributors of bitterness in beer. Humulone and cohumulone levels vary across varieties, between roughly 20% and 50% of total alpha acids. Adhumulone levels tend to be fairly constant across varieties, at about 10% to 15% of alpha acids. Pre- and posthumulone, on the other hand, play only minor roles. When hops oxidize, alpha acids, including humulone, change their molecular makeup and produce isobutryic acid, which has a distinctly rancid, sour, cheesy odor. See also adhumulone, cohumulone, and lupulone.