In pyrosequencing, molecules of the template DNA are immobilized on the surfaces of microscopic plastic beads that are deposited in small wells in a fiber-optic slide with one bead per well. A primer and DNA polymerase are added, and then a dNTP substrate is introduced. If DNA polymerase adds that nucleotide to the new DNA strand, pyrophosphate is released and trig- gers a chemical reaction involving the firefly enzyme luciferase, which gener- ates a flash of light. Solutions of each of the four dNTPs are successively washed across the immobilized DNA template, and a detector records whether light is produced in the presence of a particular dNTP. In this way, the se- quence of nucleotides complementary to the template strand can be deduced, and no electrophoretic separation is needed.