In mammals, gene dosage of X-chromosomal genes is equalized between sexes by random inactivation of either one of the two X chromosomes in female cells. In the initial phase of X chromosome inactivation (XCI), a counting and initiation process determines the number of X chromosomes per nucleus, and elects the future inactive X chromosome (Xi). Xist is an X-encoded gene that plays a crucial role in the XCI process. At the start of XCI Xist expression is up-regulated and Xist RNA accumulates on the future Xi thereby initiating silencing in cis. Recent work performed in my laboratory indicates that the counting and initiation process is directed by a stochastic mechanism, in which each X chromosome has an independent probability to be inactivated. We also found that this probability is determined by the X:ploïdy ratio. These results indicated the presence of at least one X-linked activator of XCI. With a BAC screen we recently identified X-encoded RNF12 to be a dose-dependent activator of XCI. Expression of RNF12 correlates with Xist expression, and a heterozygous deletion of Rnf12 results in a marked loss of XCI in female cells. The presence of a small proportion of cells that still initiate XCI, in Rnf12+/- cells, also indicated that more XCI-activators are involved in XCI. Here, we propose to investigate the molecular mechanism by which RNF12 activates XCI in mouse and human, and to search for additional XCI-activators. We will also attempt to establish the role of different inhibitors of XCI, including CTCF and the pluripotency factors OCT4, SOX2 and NANOG. We anticipate that these studies will significantly advance our understanding of XCI mechanisms, which is highly relevant for a better insight in the manifestation of X-linked diseases that are affected by XCI.