Summary: A GFP transgene has been integrated on the proximal part of the mouse X chromosome just distal of Timp and Syn1. During development, this X‐linked GFP transgene exhibits widespread green fluorescence throughout the embryonic and adult life of male mice but displays mosaic expression in tissues as a result of X‐inactivation in females. In living female embryos, inactivation of the transgene is imprinted in extraembryonic regions and random in the embryo proper, demonstrating that this reporter is behaving in a similar fashion to the majority of X‐linked loci, and so provides a vital readout of X chromosome activity. This is observation is further supported in T16H/X female mice harboring the GFP transgene on the normal X chromosome where reporter inactivation is observed in somatic cells. The differential expression of GFP activity facilitates fluorescence activated cell sorting for the purification of GFP+ vs. GFP‐ cells from female embryonic tissues, thereby allowing access to populations of cells that have kept active a particular X chromosome. By tracking the activity of this X‐linked GFP transgene, we discovered that the primary and secondary giant cells of the X/X placenta maintain an active paternal copy of this transgene on the presumed silenced paternal X‐chromosome. This finding implies that the imprint on the paternal X chromosome may be relaxed in these trophectodermal derivatives. genesis 29:133–140, 2001. © 2001 Wiley‐Liss, Inc.