Fig. 6. Three different paths to X inactivation. (A) In a normal X chromosome, all of the XCI_repress loci must be turned off by I. (B) In a mutant X chromosome where some of the XCI_repress loci have been removed, only a small amount of I is required to initiate XCI. Because these chromosomes have a lower threshold to overcome before initiating XCI, there is a higher probability that they will inactivate. (C) In a mutant X chromosome where all of the XCI_repress loci have been removed, XCI will initiate without any I.