In 1965, Stanley Gartler and David Linder were able to demonstrate clonality of tumors in human females using an event (X chromosome inactivation) that occurs early in development in mammalian females. X chromosome inactivation randomly silences most of the genes on one of the two X chromosomes in every cell of the embryo. The female thus becomes a mosaic for any X-linked gene for which she is heterozygous, and normal tissues are therefore composed of a nearly equal mixture of cells expressing the two different phenotypes. However, if a tumor begins from a single cell, then all the cells of the tumor should express the same X-linked allele. By examining expression of different isoenzymes of the sex-linked glucose-6-phosphate dehydrogenase (G6PD) locus in heterozygous women, Gartler and Linder demonstrated that leiomyoma tumor cells, even from cancers consisting of billions of cells, expressed only one form of the marker, whereas even small patches of normal tissue contained cells expressing both forms of the marker. This finding was consistent with the growth of a single founder cell into a tumor. The clonal origin of tumors has been confirmed many times since, initially through the work of a junior colleague Philip J. Fialkow.