Next-generation reversible terminator sequencing. (a) The reversible terminator method of sequencing uses fluorescent tags to identify nucleotides. Blocking groups on each fluorescently labeled nucleotide prevent multiple nucleotides from being added per cycle. (b) Six successive cycles from one very small part of an Illumina sequencing run. Each colored spot represents the location of an immobilized DNA oligonucleotide affixed to the surface of the flow cell. The circled clusters represent the same spot on the surface over successive cycles and give the sequences indicated. Data are automatically recorded and analyzed digitally. (c) Typical flow cell used for a next-generation sequencer. Millions of DNA fragments can be sequenced simultaneously in each of the eight channels.