The story of the gene begins in an obscure Augustinian abbey in Moravia in 1856, where a monk stumbles on the idea of a ‘unit of heredity’. It intersects with Darwin’s theory of evolution, and collides with the horrors of Nazi eugenics in the 1940s. The gene transformed post-war biology. Another intriguing narrative from The Pulitzer Prize winner Siddhartha Mukherjee.