Recent work has shown that X-chromosome inactivation is brought about by Xist mRNA, which coats the inactive X-chromosome. This paper presents a hypothesis on the function of this RNA. It is suggested that interspersed repetitive elements of the LINE type, in which the X-chromosome is particularly rich, act as booster elements to promote the spread of Xist mRNA. Contact with this RNA causes the LINE elements to be sensed as repeated elements by the cell's system for repeat-induced gene silencing. This leads to the silencing of these elements and the intervening unique sequences by their conversion to heterochromatin.