—O-Z

wherein Z is any of —C(R^IV)₂—O—R″, —C(R′)₂—N(R″)₂, —C(R′)₂—N(H)R″, —C(R^IV)₂—S—R″ and —C(R′)₂—N₃,

wherein —C(R^IV)₂—O—R″ is of the formula —CR⁴(R⁵)—O—CR⁴(R⁵)—OR⁶ or of the formula —CR⁴(R⁵)—O—CR⁴(R⁵)—SR⁶; and wherein —C(R^IV)₂—S—R″ is of the formula —CR⁴(R⁵)—S—CR⁴(R⁵)—OR⁶ or of the formula —CR⁴(R⁵)—S—CR⁴(R⁵)—SR⁶;

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;

each R⁴ and R⁵ is independently a hydrogen atom or an alkyl group;

R⁶ is alkyl, cycloalkyl, alkenyl, cycloalkenyl or benzyl; and

wherein said molecule may be reacted to yield an intermediate in which each R″ is exchanged for H, which intermediate dissociates under aqueous conditions to afford a molecule with a free 3′OH; with the proviso that where Z is —C(R^IV)₂—S—R″, both R^IV groups are not H.