The invention generally relates to nucleotide analogs and methods of their use in sequencing-by-synthesis reactions. In certain embodiments, the invention provides a nucleotide analog including a detectable label attached to a nitrogenous base portion of a nucleotide analog by a cleavable linker, in which contact of the analog with at least one activating agent results in cleavage of the label and elimination of the linker, thereby producing a natural nucleotide, a 9-deaza-G, 9-deaza-A, or ψ-uridine.