Analysis of X-chromosome inactivation patterns can be a useful tool in the identification of carriers of certain X-linked diseases and also for other investigations, such as gene mapping and clonality analysis. X inactivation is the process in females whereby one of the two X chromosomes of a cell is maintained in an inactive state with most genes remaining untranscribed. It occurs early in embryogenesis at the late blastocyst stage and normally is random such that for each cell there is an equal chance of either X chromosome being inactive. Once a cell has undergone X inactivation, all cells originating from it maintain the same chromosome inactive throughout life. Adult tissues therefore are normally a mosaic of groups of cells with the same X chromosome inactive.