5-Azadeoxycytidine distinguishes between active and inactive X chromosome condensation.

Treatment with 5-azadeoxycytidine (5-aza-dC) causes dramatic inhibition of mitotic condensation in the inactive X chromosome, without affecting the active X chromosome and autosomes. The undercondensed chromatin structure is most prominent in the late-replicating regions of the inactive X chromosome, whereas specific earlier-replicating segments on the inactive X probably associated with large blocks of genes that escape inactivation remain normally condensed. This segmentation of inactive X chromosomes has been evolutionarily conserved and is unique to mammals. It suggests temporal or mechanistic separation in condensation of inactive X chromatin. Condensation of chromatin structure is a novel and general feature of inactive X chromosomes that may be involved, directly or indirectly, in the maintenance and stabilization of X inactivation.