Abstract: Activating groups for cyanine dyes used to label chain terminators in nucleotide sequencing, based on N-hydroxyphthalimide, are disclosed. From these activating groups, activated dyes of the present Invention are prepared which react with the derivitized nucleotide chain terminators to give a labeled chain terminator of the present Invention. The activating groups of the present Invention allow the dye-chain terminator reaction to occur at a much higher yield and with much greater selectivity for the mono-substituted product, compared with the prior art.