Phytocannabinoids, synthases, genotypes and chemotypes of Cannabis. Phytocannabinoids are synthesised via a multi-step pathway involving different enzymes. The precursor cannabigerolic acid (CBGA) is first synthesised by a prenyltransferase from the precursor molecules geranyl pyrophosphate (GPP) and olivetolic acid (OA). CBGA is metabolised into tetrahydrocannabinolic acid (THCA) via THCA synthase, into cannabidiolic acid (CBDA) via CBDA synthase or cannabichromenic acid (CBCA) via CBCA synthase. The different synthases are encoded by the BT (encoding for an active THCA synthase) and BD (encoding for an active CBDA synthase) loci. BT/BT plants produce mainly THCA (chemotype I), while BD/BD plants produce predominantly CBDA (chemotype III). Presence of BT and BD results in chemotype II (THCA and CBDA intermediate). B0indicates that only non-functional THCA and CBDA synthases are present, which results in the accumulation of CBGA (chemotype IV). Cannabis varieties with very low overall levels of cannabinoids are categorized chemotype V, which is caused by a homozygous recessive allele of locus O. To complicate matters further, there is also a locus C, which is encoding for CBCA synthase. However, in almost all varieties, CBCA is only produced in young immature flowers. Chemotypes I and II can be considered marijuana, while the other low-THC chemotypes can be considered hemp varieties of Cannabis.