Robert Martienssen has made major discoveries relating to the way plants control the expression of their genes. Working with maize, yeast and the weed Arabidopsis, he focuses on the chemical modifications to DNA that determine which genes are active — a process known as epigenetics. Robert’s work explains the effect on plants of ‘jumping genes’, or DNA transposable elements, reported in 1951 by Barbara McClintock, whom he worked alongside early in his career. He discovered that small pieces of RNA, in association with proteins of the Argonaute family, silence transposons in seeds so that gene expression remains stable from one generation to the next. His work was cited by the journal Science as part of its ‘Breakthrough of the Year: 2002’ feature on small RNAs. He has extended his epigenetic studies from seeds to pollen, and his discoveries have implications for plant breeding — including hybrid cloning — and the development of biofuels.