Sex differences in disease -- Evolution of the human sex chromosomes and a portrait of the human X -- X chromosome dosage compensation : an overview -- The discovery of X chromosome inactivation -- Experimental models for X inactivation studies -- Theme 1 : the initial step : creating the active and inactive X -- Theme 2 : subsequent steps : spreading and maintaining inactivation -- Variations 1 : evolution of the X inactivation center -- Variations 2 : stability of the inactive X -- Variations 3 : choice of active X -- The single active X -- Mosaicism -- Epimutations, chromatin disorders and sex differences in phenotype -- Determinants of female phenotypes.