Fig. 1.

Diagrammatic summary of the method. (A) Identify sites of transcription using RNA-FISH by targeting intronic RNA. (B) Resulting images are of nuclei that contain diffraction-limited spots (∼200 nm). Desired measurement resolution is on the order of tens of nanometers. Increased resolution is attained by localizing signals with high precision (∼20 nm) through the mapping of the precise location of the point source of light within the image of a diffraction-limited spot. Spots in each channel are isolated from the larger image for localization. (C) Distances between signals can then be assessed with high resolution. Bar: 2 μm.