The B-side of .

Female mammals express the long noncoding X inactivation-specific transcript ( ) RNA to initiate X chromosome inactivation (XCI) that eventually results in the formation of the Barr body. encompasses half a dozen repeated sequence stretches containing motifs for RNA-binding proteins that recruit effector complexes with functions for silencing genes and establishing a repressive chromatin configuration. Functional characterization of these effector proteins unveils the cooperation of a number of pathways to repress genes on the inactive X chromosome. Mechanistic insights can be extended to other noncoding RNAs with similar structure and open avenues for the design of new therapies to switch off gene expression. Here we review recent advances in the understanding of and on this basis try to synthesize a model for the initiation of XCI.