XX meetings

An interphase meeting between X chromosomes, revealed by Na Xu, Chia-Lun Tsai, and Jeannie Lee (Harvard Medical School, Boston, MA), ensures that one and only one is silenced.

Silencing of one of the two X chromosomes in a female somatic cell brings the gene dosage level down to that of male cells. Inactivation is controlled by several noncoding RNAs transcribed from, and acting in cis upon, the X inactivation center (XIC). But the field has been perplexed as to how one chromosome knows what the other is doing to keep inactivation mutually exclusive.

The new results suggest that a prior meeting between X chromosomes sets the decision. Although mammalian chromosomes normally only pair during meiosis, the authors saw transient contact between X chromosomes just before the inactivation of one.

Pairing...