Numerous sense-antisense gene pairs have been discovered in various organisms. 
Antisense genes play important roles in establishing parentally imprinted gene 
expression patterns in mammals. Typically, protein-coding sense genes are 
reciprocally regulated by their non-coding antisense partners. One example for 
antisense regulation is the Xist (X-inactive specific transcript) and Tsix gene 
pair, which is pivotal in X-inactivation. Xist works as a functional RNA 
molecule that recruits repressive chromatin factors towards one of the female Xs 
for inactivation. Antisense Tsix transcription negatively regulates Xist and 
protects one X-chromosome in cis from inactivation by Xist. Albeit, the precise 
molecular mechanism is still obscure it has been shown that Tsix transcription 
regulates the chromatin structure by altering histone tail modifications and DNA 
methylation at the Xist promoter. In addition, Xist and Tsix RNA form an RNA 
duplexes in vivo and are processed to small RNAs, which have a potential 
regulatory function. Here we review the latest findings and based on ample 
experimental data consider models for antisense-mediated gene regulation in 
X-inactivation.