Fig. 4. Double immunolabelling. (A) Double-labelling with mouse monoclonal anti-HPC2 antibody (15 nm gold particles) and rabbit anti-BMI1 antibody (10 nm gold particles, some indicated by arrowheads) in an SW480 cell stained with osmium ammine specific for DNA; most signal is observed on the periphery or outside DNA-rich regions. (B) Double labelling experiment with PcG protein BMI1 (6 nm gold particles) and nascent BrU-containing RNA (15 nm particles) in a T24 cell; EDTA staining. Both labels mostly occur near the border of condensed chromatin regions (c), sometimes in association with perichromatin fibrils (some indicated by arrowheads). A cluster of interchromatin granules (large arrow) exhibits anti-PcG protein label only on its periphery (small arrows). Bars, 0.2 µm.