is and deprotection conditions; Xl is either O, S, NH or NNH; D is a protecting group which may be removed to permit X1 to be coupled to the phosphorus group of a second nucleotide; each X3 is independently a linker-arm joined at a first end to R2 and joined at a second end to M, wherein each X3 is stable to DNA synthesis and deprotection conditions; each M is independently either a label stable t