At least two hundred different families of sesquiterpenoid compounds have been identified in plants. Although related isoprenoid compounds such as giberellins and carotenes are essential to survival, most cotton sesquiterpenes are classified as secondary metabolites. Many have antibiotic activity and are synthesiszed following infection with pathogens, so are considered to be phytoalexins. Representatives of a number of sesquiterpene families are present in cotton, including several with different cyclic structures. Different cyclic sesquiterpenes originate from folding of FPP or its isomers into specific configurations. However, since a single synthase may lead to the formation of several products through variable patterns of condensation, the number of unique cyclases present is undetermined. Further difficulties in identifying and cloning specific synthase genes arise because there are regions of amino acid sequence homology between mono- sesqui- and di- terpene cyclases, and from the fact that some or all of the cyclases are coded by members of small gene families present in the genome.