Jean Thomas is Professor of Macro-Molecular Biochemistry at the University of Cambridge. Her discovery of how histones are associated led directly to the formulation by Kornberg of the universal nucleosome model for the structure of chromatin. She was the first to isolate and characterize the histone octamer, which was subsequently used in image reconstruction studies to generate the first correct model of the nucleosome. She has made significant contributions to understanding the chromosomal proteins HMG1 and 2, which are believed to play a general ‘architectural’ role in recombination, repair and transcription. Since chromosome structure is central to events such as recombination, transcription, mutation, breakage and repair, this work is fundamental importance to biomedical science.