The mouse X -chromosome controlling elements, detected by their influence on the position effect variegation caused by the X -autosome translocation T (1; X ) Ct , have been found to modify the heterozygous phenotypes of two X -linked genes. It is proposed that X -inactivation can be incomplete, the level of inactivation or the frequency of cells in which inactivation is incomplete being dependent upon the ‘state’ of the controlling element located in the X . The data suggest that this is a consequence of a reversal, or partial reversal, of inactivation of the X as a whole in some cells rather than a vairable spread of inactivation along the length of the X .