3′-(O-Propargyl)-ATP can be incorporated in RNA at the 3’end using poly(A) Polymerase. The added alkyne can now be modified by Click Chemistry by reactions with azides. This is highly beneficial, as labeling of mRNA with this approach is modular and not limited to enzymatic compatibility of bulky pre-labeled nucleotides. Moreover, only a very limited number of such pre-modified nucleotides, mainly dye-labeled ones, are commercially available.