X chromosome inactivation begins when a novel chromosomal RNA (cRNA) from the 
imprinted mouse Xist or human XIST locus coats or "paints" one X chromosome in 
cis and initiates a cascade of chromosome remodeling events. Molecular 
cytological studies have proven invaluable for understanding the distinctive 
cellular behavior of this singular RNA involved in chromosome structure and 
regulation. While the detailed mechanism of XIST/Xist (X-inactivation Specific 
Transcript) RNA function remains largely unknown, recent advances provide new 
insights into the complex cellular factors which impact the RNA's localization 
to the chromosome, as well as the early events of chromosome remodeling that 
follow painting by Xist RNA. Because chromatin changes can be directly 
visualized on a silenced chromosome, X chromosome inactivation provides an 
advantageous model to investigate genome-wide heterochromatin formation and 
maintenance, with wide-ranging implications for normal cells and disease.