Mary Frances Lyon is an eminent British geneticist noted for her recognition of a phenomenon known as X-chromosome inactivation, a fundamental genetic control mechanism. Also referred to as "Lyonization," X-chromosome inactivation involves the random silencing of one of the two X chromosomes in each of the cells of female mammals. Dr. Lyon's groundbreaking work in mouse genetics has had great importance for human medicine, providing insights into such X-linked inherited diseases as hemophilia, Duchenne muscular dystrophy, fragile X syndrome, and certain cancers. Dr. Lyon has also made major contributions to understanding the risks of gene damage associated with radiation and other environmental mutagens.