A three-factor model can explain symmetry breaking and the n-1 rule during X-chromosome inactivation

A three-factor mathematical model shows that stochastic Xist loading plus SPEN–Xist feedback can explain symmetry breaking and the n-1 rule in X-chromosome inactivation, yielding one active X per diploid genome.

• Why it matters: The key gap is how cells count X chromosomes to silence while leaving the correct number active, since the mechanisms for chromosomal counting during XCI are not fully understood.
• What they did: The model integrates Xist, an X-linked transcriptional activator of Xist, and autosomally encoded SPEN, using stochastic differences in initial Xist–chromosome association and coupled positive/negative feedback to propagate SPEN–Xist assemblies.
• The result: The framework accounts for all known chromosomal counting scenarios during XCI and implies that asymmetric SPEN–Xist propagation can drive symmetry breaking and chromosomal counting, with potential relevance to autosomal gene regulatory circuits.

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