This Centers for Disease Control and Prevention (CDC) scientist was performing one of the last steps in a poliovirus testing process. Dyes are used to determine, which of the nucleotides (basis of genetic material) are 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.