Controlling elements in the mouseX-chromosome III. Influence upon both parts of anXdivided by rearrangement

SUMMARY Data are presented which support the conclusion that in the flecked translocation, T (1; X) Ct , there is a spread of inactivation into both sides of the autosomal region inserted into the X . This would indicate that both parts of the divided X are subject to the X -inactivation process. The data also demonstrate that the inactivation of autosomal genes lying near each end of the insertion are modified by the X -chromosome controlling element system, Xce . Since the element modifies the heterozygous expression of X -linked genes on one side of the insertion, it would therefore be expected that it similarly modifies the heterozygous phenotypes of those on the other side. The data thus support the concept that the controlling element is the master gene, receptor site or inactivation centre which regulates the X inactivation process.