Movement of the inactivating chromosome to the heterochromatic compartment is coincident with the irreversibility of the inactivation process.

X chromosome inactivation is initiated by expression of Xist RNA and occurs through the formation of facultative heterochromatin. Inactivation is necessary to achieve dosage compensation in female mammals. Previous studies have shown that Xist-mediated chromosome inactivation becomes irreversible after 72 hours of ES cell differentiation. Using fluorescent in-situ hybridization and a male mouse ES cell line transgenic for Xist on chromosome 11, we have now shown that during this important time period the inactivating chromosome moves into the heterochromatic compartment of the nucleus