Transcriptional activity of an X-borne gene and its Y paralogue in male and female fibroblasts. The HUWE1Y probe (red signal) detects the paralogue located on the Y, and the HUWE1X probe (green signal) detects the paralogue on the X chromosome. (a) Male nucleus with transcription from the single X locus (HUWE1X, green) and the single Y locus (HUWE1Y, red). Different signal intensities from different probes does not correlate to transcription level. (b) Female fibroblast nuclei with transcription from one (left) and two (right) X-borne loci (HUWE1X, green), and no expression detected with the Y-specific probe (HUWE1XY, red). Nuclei are counterstained with DAPI (blue).