This Centers for Disease Control and Prevention (CDC) scientist is performing one of the last steps in poliovirus testing process. Dyes are used to determine which of the nucleotides (basis of genetic material) is present in a sequence. The outer coat of the virus (the capsid) is sequenced to find out where the virus has circulated. This machine is a sequencing instrument, a Sanger sequencer. Sanger sequencing implements a traditional type of sequencing. The plates go in, and the machine analyzes the reactions. Each base has a different color. The machine processes the bases, and sorts them, producing a chromatogram of four colors. See PHIL 22909 for an example of this four-color chromatogram. Sequencing gives CDC a window to where a virus has been, and how long it has been circulating. This allows CDC to focus our vaccination programs on specific regions inside pertinent countries.