Fig. 2. Stochastic model of X chromosome inactivation. (A) The various components of the stochastic model. (B,a) Prior to cell differentiation, the stochastic model proposes that the autosomes produce a trans-acting factor (purple stars) that induces Tsix expression on both X chromosomes (blue shape). (b) Tsix, when bound by autosomal trans-acting factors, represses Xist transcription (red shape). (c) After differentiation, an X-linked locus (yellow triangle) produces a trans-acting factor (red circles) that attempts to upregulate Xist. (d) Competition between Tsix- and Xist-promoting factors creates a probability for each X chromosome to inactivate. Cells that do not inactivate either X chromosome will continue to produce the factor that promotes Xist upregulation in subsequent cell cycles. Cells that inactivate both X chromosomes will either die, or reactivate one. The percentages shown here reflect the proportions of each XCI configuration observed 7 days after differentiation. Xa, active X chromosome; Xi, inactive X chromosome.