A novel domain in the mouse X-chromosome that escapes X-inactivation. Results from sequential RNA-DNA FISH experiments on the novel female-biased cluster containing 5530601H04Rik and 2610029G23Rik, and on Xist, in female mouse cells are shown. A: Confocal images, showing monoallelic Xist RNA signals (red) and biallelic RNA signals from the novel female-biased cluster (green). In agreement with the expected genomic distance between the Xist locus and the novel cluster (~1.6Mbp), DNA signals from Xist (yellow) appear close to the DNA signals from the novel cluster (white). RNA and DNA signals for the novel cluster are proximal, indicating that the intended nascent RNA was targeted. The nucleus is stained with DAPI (blue). B: Enlargements of the merged RNA + DNA image in panel A, showing that RNA and DNA signals for the novel cluster are overlapping. C: Quantitative data showing allelic expression of the novel escapee cluster in nuclei positive for Xist RNA (n = 88).