Fig. 5. The feedback model of X chromosome inactivation. (A) A description of the components required for the model. (B,a) The active X chromosomes produce a trans-acting signal, A, that saturates specific sites on the autosomes. (b) Once saturated, the autosomes produce a swarm of inactivation signals, I. These signals bind to each other and to XCI inhibitors on the X chromosomes. Once all of the XCI inhibitors on an X chromosome are sufficiently bound by I, the XCI initiator induces inactivation. (c) With only one active X chromosome producing A, the autosomes are no longer saturated with A and stop producing I.