Maybe you've had this scenario described to you by a customer: About an hour after dinner she begins to notice an uncomfortable sensation in her abdomen. She says the pain is cramplike, builds in intensity and shoots back toward her right shoulder blade. Nausea follows. She takes an antacid but wonders whether she has symptoms of appendicitis, food poisoning or an ulcer. A visit to her doctor reveals that she is actually suffering from complications of cholelithiasis, also known as gallstones.
Sterol lipids, such as cholesterol and its derivatives, are an important component of membrane lipids,  along with the glycerophospholipids and sphingomyelins. The steroids , all derived from the same fused four-ring core structure, have different biological roles as hormones and signaling molecules . The eighteen-carbon (C18) steroids include the estrogen family whereas the C19 steroids comprise the androgens such as testosterone and androsterone . The C21 subclass includes the progestogens as well as the glucocorticoids and mineralocorticoids .  The secosteroids , comprising various forms of vitamin D , are characterized by cleavage of the B ring of the core structure.  Other examples of sterols are the bile acids and their conjugates,  which in mammals are oxidized derivatives of cholesterol and are synthesized in the liver. The plant equivalents are the phytosterols , such as β-sitosterol , stigmasterol , and brassicasterol ; the latter compound is also used as a biomarker for algal growth.  The predominant sterol in fungal cell membranes is ergosterol . 
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