Analysis of a section of FMD virus genome in a seqencing gel. The four lanes correspond to different dNTP mixtures, and each contains segments of different length. Every labelled segment (band) in a particular lane is terminated by the same base. Segments terminated by all four bases (each type of base in its own lane) are thus present in the gel, and examination of the relative longitudinal positions of the bands in the set of four indicates directly the nucleotide sequence.