Fig. 4. Xist is stably repressed in the absence of spliced Tsix RNA. Transcriptional activity of the Xist locus was analyzed by RT-PCR using RNA isolated from E13.5 male and female embryos carrying the respective mutation. Diagrams show the structure of each mutant allele and positions of primers used for PCR. Transcription of the Xist locus on the mutated X was examined using a primer set specific for EGFP.