When ribosome biogenesis is active, the conWnement of certain machineries in the FCs, DFC or GC makes it possible to reveal these subnucleolar constituents by immunoXuorescence as illustrated for FCs ( Fig. 2A) , DFC (Fig. 2Ba , b), and GC (Fig. 2Bc, d) . The factors associated with the rDNA transcription machinery are distributed in several foci, most frequently inside the nucleolar volume as illus-trated for UBF. These foci correspond to FCs. A distribution within the network inside the nucleolus is typical of the DFC as demonstrated for Wbrillarin. Labeling of the nucleolar volume excluding small areas contained within the volume is typical of the GC as illustrated for B23/NPM. These labeling patterns (FCs, DFC, GC) in the nucleoli provide a good indication of the step of ribosome biogenesis concerned and also reveal the blockage of ribosome biogenesis when this organization is disturbed (see below).