A steroid is an organic compound with four rings arranged in a specific configuration, such as dietary lipid cholesterol, estradiol, testosterone and dexamethasone. Steroids function as important components of cell membranes and many of them are signaling molecules. The core structure of the steroid is composed of seventeen carbon atoms, bonded in four fused rings: three six-member cyclohexane rings and one five-member cyclopentane ring. Hundreds of steroids are found in plants, animals and fungi.