X-inactivation has several biologically curious aspects. Whether a cell initiates X-inactivation depends on a counting mechanism that allows one X to remain active per cell.
Coating is associated with chromosome silencing.
All of these events are cis-limited, so the events occurring on one X does not influence the other X or any other chromosome in the cell. By deletional analysis using YAC and P1 technologies, we subsequently showed that X-inactivation can be reduced to a subregion containing only 80 kb of sequence including Xist. When placed ectopically on an autosome, the autosome behaves like an X and recapitulates X-inactivation. Together with other published observation, our data suggests that the 80 kb sequence carries multiple functional domains. These include one required for somatic Xist expression and a negative cis-regulator of Xist.