In 1949, Canadian physician and medical researcher Murray Barr (1908–1995) noticed a dark body in the neurons of female cats. It was later identified as a structure found only in the nucleus of females. It was named a Barr body in honor of its discoverer. The Lyon hypothesis refers directly to a Barr body. It was proposed by English geneticist Mary Frances Lyon (1925–) in 1961 that a Barr body is actually an inactivated X chromosome. According to this hypothesis, female mammals sequester one X chromosome in each of their cells during the early stages of development. This folded chromosome becomes the dark body of Barr’s observation. This means that both males and females rely on the information from only a single X chromosome. Therefore, it is only one X chromosome that provides genetic information in both males and females.