Before massively parallel machines, DNA sequence was read one fragment at a time. Sanger sequencing uses chain-terminating dideoxynucleotides to generate nested sets of DNA strands that end at every possible position. When the fragments are separated by size, the order of bases can be deduced.

Each of the four terminators carries a different fluorescent dye. A capillary electrophoresis instrument detects the color of the final base as fragments pass a laser, producing a chromatogram of colored peaks. The method is highly accurate for reads up to roughly a thousand bases.

Although next-generation platforms now dominate large projects, Sanger sequencing remains the gold standard for validating individual variants and sequencing short targeted regions. Many clinical laboratories still rely on it for its reliability and straightforward interpretation.

• Classic chain-termination sequencing method
• Uses fluorescent dideoxynucleotides
• Produces accurate reads of moderate length
• Still used for validation and targeted analysis