As indicated in the Figure above showing the pathway of cholesterol biosynthesis a molecule of geranylpyrophosphate (GPP) and a molecule of isopentenylpyrophosphate (IPP) are condensed into farnesylpyrophosphate (FPP) through the action of the farnesyl diphosphate synthase enzyme which is encoded by the FDPS gene. Through the action of the ER-localized enzyme, dehydrodolichyl diphosphate synthase (encoded by the DHDDS gene), farnesylpyrophosphate is elongated via the sequential head-to-tail addition of multiple isopentenylpyrophosphate groups in a reaction referred to as cis -prenylation. The number of IPP substrates added ultimately determines the overall number of isoprene units in dolichol which in humans ranges from 17 to 21. The DHDDS gene is located on chromosome and is composed of 10 exons that generate five alternatively spliced mRNAs each of which encode a distince protein isoform. The product(s) of the DHDDS reaction is referred to as a polyprenolpyrophosphate. The pyrophosphate is removed by an as yet uncharacterized enzyme activity that may be either a polyprenol pyrophosphate phosphatase or a polyprenol phosphatase resulting in the formation of a polyprenol.