Abstract: The invention provides modified nucleotide or nucleoside molecule comprising a purine or pyrimidine base and a ribose or deoxyribose sugar moiety having a removable 3?-OH blocking group covalently attached thereto, such that the 3? carbon atom has attached a group of the structure —O—Z wherein Z is any of —C(R?)2-O—R?, —C(R?)2-N(R?)2, —C(R?)2-N(H)R?, —C(R?)2-S—R? and —C(R?)2-F, wherein each R? is or is part of a removable protecting group; each R? is independently a hydrogen atom, an alkyl, substituted alkyl, arylalkyl, alkenyl, alkynyl, aryl, heteroaryl, heterocyclic, acyl, cyano, alkoxy, aryloxy, heteroaryloxy or amido group, or a detectable label attached through a linking group; or (R?)2 represents an alkylidene group of formula ?C(R??)2 wherein each R?? may be the same or different and is selected from the group comprising hydrogen and halogen atoms and alkyl groups; and wherein said molecule may be reacted to yield an intermediate in which each R? is exchanged for H or, where Z is —C(R?) 2-F, the F is e