In the article "Numbers and Organization of RNA Polymerases, Nascent Transcripts, and Transcription units in HeLa Nuclei," by D.A. Jackson, F.J. Iborra, E.M.M. Manders, and P.R. Cook (Mol. Biol. Cell 9, 1523-1536, 1998), an error was made in calculating the number of transcription sites in a nucleus from numbers seen in two-dimensional sections. The measured numbers of clusters per µm² given in Tables 1 and 2 are correct, but the calculated number of sites per nucleus given in RESULTS is incorrect. One cluster per µm² corresponds to 5500 extranucleolar sites per cell (not 2400, as originally stated). This means that each site would contain ~14 active polymerases (not ~30, as originally stated). The central conclusion of the articlenamely that many active polymerases are concentrated in each siteremains unaffected by this correction.