Clinical molecular geneticists studying a radiograph of a DNA sequence, 2000.

Clinical molecular geneticists studying a radiograph of a DNA sequence, Great Ormond Street Hospital, London, May 2000. A gene is an instruction written in the language of the DNA code, which has four chemical letters (bases): 'A', 'C', 'T' and 'G'. Each 'A' is alway paired with a 'T', and each 'G' with a 'C'. Scientists use a technique called DNA sequencing to reveal the order of these base pairs.