(a) For all parameter sets that reproduced mono-allelic Xist up-regulation in the cXR-tXA model, the maximal fraction of cells with bi-allelic Xist expression observed during the simulation is shown as a function of the the ratio of switch-ON time (first time point, when Xist levels reach 20% of the high steady state) and tXA silencing delay (siltXA). If Xist up-regulation is slow (high Switch-ON time), it will normally occur one allele at a time. Subsequent silencing will shift the system to the bistable regime (cp. Fig. 2e) and thereby lock in the mono-allelic state before Xist up-regulation from the other X chromosome occurs. This results in a low frequency of bi-allelically expressing cells as observed in mice. If Xist up-regulation is rapid and silencing is slow (long silencing delay siltXA), Xist will initially be expressed from two alleles as observed in rabbit embryos. In this scenario the choice of the inactive X can subsequently occur through mono-allelic silencing of tXA and cXR. Alternatively, silencing of both alleles might reverse Xist up-regulation completely as Xist expression is unstable if both tXA alleles are silenced such that the cell can undertake a second attempt to reach the mono-allelic state. (b) Simulation of bi-allelic expression upon reduced Xist-mediated silencing as observed in human embryos, assuming that in the first 4 days of the simulation either silencing and cXR expression is absent (left) or that cXR is silenced partially (dampening), while tXA is unaffected by Xist (right). Boxplots show the percentage of mono- and bi-allelically expressing cells for 100 randomly chosen parameter sets that can reproduce mono-allelic Xist up-regulation (center line, median; box limits, upper and lower quartiles; whiskers, most extreme data points not considered outliers; points, outliers).