the sugar nucleotides may be mono di tri or cyclic phosphates the modified nucleotides or nucleosides of the invention comprise the guanine base sugar and one or more phosphate groups if appropriate and a detectable label comprising a fluorophore the detectable label is attached to the guanine base through a linking group in the modified nucleosides and nucleotides of the invention the linking group comprises a polyethylene glycol spacer although other suitably hydrophilic groups of similar length to the polyethylene glycol spacers of the invention approximately 5 to 150 atoms may be used as an alternative to polyethylene glycol spacers preferably the spacer comprises between 5 and 30 ethylene oxide groups u2014 ch 2 2 o u2014 still more preferably between 10 and 25 ethylene oxide groups exemplified herein and thus particularly preferred ethylene oxide spacer groups are u2014 ch 2 2 o 11 u2014 and u2014 ch 2 2 o 23 u2014 preferably u2014 ch 2 2 o 23 u2014 the linking groups used in the present invention serve to space the fluorescent label away from the guanine base such that the amount of quenching of the fluorescent signal from the fluorophore by the guanine base is reduced or substantially eliminated as compared to a nucleotide or nucleoside of analogous structure but lacking the linking group at the same time the fluorophore is maintained in indirect covalent attachment with the remainder of the nucleotide nucleoside the nature of the fluorophore present in the fluorescent label is generally not limited it may be any fluorophore compatible with labelling.