Definition: Generic term for a class of carotenes and their oxygenated derivatives (xanthophylls) consisting of 8 isoprenoid units (thus, tetraterpenes) joined so that the orientation of these units is reversed at the center, placing the two central methyl groups in a 1,6 relationship in contrast to the 1,5 of the others. All carotenoids may be formally derived from the acyclic C40H56 structure known as lycopene, with its long central chain of conjugated double bonds by hydrogenation, dehydrogenation, oxidation, cyclization, or combinations of these. Included as carotenoids are some compounds arising from certain rearrangements or degradations of the carbon skeleton, but not retinol and related C20 compounds. The nine-carbon end groups may be acyclic with 1,2 and 5,6 double bonds or cyclohexanes with a single double bond at 5,6 or 5,4 or cyclopentanes or aryl groups; these are now designated by Greek letter prefixes preceding “carotene” (α and ΄, which are used in the trivial names α-carotene and ΄-carotene, are not used for that reason). Suffixes (-oic acid, -oate, -al, -one, -ol) indicate certain oxygen-containing groups (acid, ester, aldehyde, ketone, alcohol); all other substitutions appear as prefixes (alkoxy-, epoxy-, hydro-). ). The configuration about all double bonds is trans unless cis and locant numbers appear. The prefix retro- is used to indicate a shift of one position of all single and double bonds; apo- indicates shortening of the molecule. Many carotenoids have anticancer activities.