Mary Frances Lyon. 15 May 1925—25 December 2014

Mary Lyon was a central figure in twentieth-century mouse genetics. She is best known for the phenomenon that bears her name, ‘Lyonization’ or X chromosome inactivation, the process whereby one of the two X chromosomes in female mammals becomes heterochromatic and genetically inert, resulting in dosage compensation for X-linked genes between XX females and XY males. Her work on the t -complex, developing the concept of responder and distorter loci in the mouse, led to fundamental insights into non-Mendelian inheritance in mammals and exemplifies her exceptionally clear thinking. Her broader work in mouse genetics helped to lay the foundations for the widespread use of the mouse as an essential model organism in molecular medicine, cell and developmental biology and deciphering the function of the human genome.