Hexaprenyl-diphosphate synthase ((2E6E)-farnesyl-diphosphate specific) (EC 2.5.1.83 HexPS hexaprenyl pyrophosphate synthetase hexaprenyl diphosphate synthase) is an enzyme with system name (2E6E)-farnesyl-diphosphate:isopentenyl-diphosphate farnesyltranstransferase (adding 3 isopentenyl units).