Figure 1. Mechanism of cDNA generation from chemically ligated tags using relay primer-mediated bypass on a photochemically ligated non-traversable template. A terminal primer is annealed to the 3′-terminus of the chemically ligated oligonucleotide and a second 5′-monophosphorylated primer (relay primer) is annealed across the chemical ligation junction. Extension of both primers and ligation generates full-length cDNA including potentially encoding sequences from both upstream and downstream of the photochemical ligation junction.