B23 (also designated NPM) is a multifunctional protein, abundant in the GC of the nucleolus that undergoes diVerent phosphorylation events during the cell cycle. It was recently demonstrated by FRAP that the kinetics of B23 depends on its phosphorylation status (Negi and Olson 2006) . During interphase, the half-time (t 1/2 ) recovery of B23 is 22 s in nucleoli but when the CK2 phosphorylation site is mutated (S125A) the t 1/2 increases to 44 s, and when a mutant mimicking phosphorylation charges of the four sites of mitotic CDK1 phosphorylation, the t 1/2 decreases to 12 s. This could indicate that the S125A-B23 protein has a higher aYnity for the nucleolar components (Negi and Olson 2006) . Alternatively this could correspond to a decreased turner-over in the nucleolar complexes in correlation with the disconnection of the DFC and GC occurring by overexpression of S125A-B23 (Louvet et al. 2006) . Overexpression during interphase of B23 mimicking four sites of mitotic phosphorylation increased the mobility of B23. It is tempting to propose that this results from a defect of aYnity for rRNAs of these B23s as demonstrated for mitotic phosphorylation of B23 (Okuwaki et al. 2002) .