Nucleoside analogues or base analogues having structure (I), where X=O or NH or S, Y=N or CHR6 or CR6, W=N or NR6 or CHR6 or CR6 or S, n=1 or 2; each R6 is independently H or O or alkyl or alkenyl or alkoxy or aryl or a reporter moiety; where necessary (i.e. when Y and/or W is N or CR6 where R6 is not O) a double bond is present between Y and W or W and W, and Q is H or a sugar or a sugar analogue or a nucleic acid backbone or backbone analogue.