DNA

DNA is a twisted ladder of two strands whose rungs are pairs of four bases (A, T, C, G); the order of those bases encodes about 20,000 genes plus the switches that control them. Every time a cell divides it must copy all three billion letters, and copying errors, radiation, tobacco chemicals and other insults introduce mutations that accumulate over a lifetime. Sequencing a tumour's DNA reveals which genes are broken and often which drug might work; sequencing DNA fragments in blood (ctDNA) lets doctors track the tumour without a biopsy.