Fig. 1. Three models of X chromosome inactivation. (A) The blocking factor model. (a) Diploid autosomes work together to create a single blocking factor (blue shape), which can bind to only one X chromosome, preventing it from inactivating. (b) Nicodemi and Prisco used computer simulations to show that if the autosomes produce a swarm of blocking factors (blue dots) that can bind to each other and to the X chromosome, then all of the blocking factor molecules will accumulate on a single X. (B) The two factor model. Autosomes produce blocking factors and X chromosomes produce transacting competence factors (red shape). Blocking factors bind to competence factors with a two to one stoichiometry and then bind to one X chromosome; the remaining competence factor binds to the other X, which inactivates. (C) The sensing and choice model. After cells start to differentiate, the two Xpr regions (yellow) pair and upregulate Xist (red) on both X chromosmes. The Tsix/Xite region (blue) pairs and chooses which chromosome to inactivate, and represses Xist on the other. The X chromosome that represses Xist remains active (green), and the other becomes inactive (red).