X chromosome inactivation, which causes functional mosaicism in females, is controlled by LINE-1 retrotransposons ("long interspersed nuclear elements"), which are concentrated in high numbers on the X chromosome in the region of the X inactivation center (XIC).
In genetics, X chromosome inactivation is a process in which an X chromosome is completely or largely silenced, so that no more gene products are created from that chromosome. Females carry two X chromosomes in each cell and would therefore have double the gene dosage compared to males.
Random inactivation of either the paternal X chromosome (Xp) or maternal X chromosome (Xm) in females serves to dose compensate for X-linked genes between the two sexes, so that female cells have the same gene dose as male cells with only one X chromosome. By randomly inactivating one of the two X chromosomes, females represent a mosaic of two cell populations, cells with active Xm or active Xp. This special epigenetic mosaic contributes significantly to the fact that women have a 6 to 7 years longer life expectancy compared to men. Thus, women can react more elastically to environmental stresses, since they basically have two functionally different cell populations due to X inactivation.