Martin G R, Epstein C J, Travis B, Tucker G, Yatziv S, Martin D W, Clift S, Cohen S
Nature. 1978 Jan 26;271(5643):329-33. doi: 10.1038/271329a0.
Evidence is presented that both X chromosomes are genetically active in clonal cultures of undifferentiated female mouse teratocarcinoma stem cells derived from a spontaneous ovarian tumour. As the cells differentiate in vitro one of the X chromosomes becomes inactivated.