NF-Y is a trimeric activator with histone fold -HFM- subunits that binds to the CCAAT-box and is required for a majority of cell-cycle promoters, often in conjuction with E2Fs. In vivo binding of NF-Y is dynamic during the cell-cycle and correlates with gene activation. We performed immunofluorescence studies on endogenous, GFP- and Flag-tagged overexpressed NF-Y subunits. NF-YA, NF-YB are nuclear proteins. Unexpectedly, NF-YC localizes both in cytoplamatic and nuclear compartments and its nuclear localization is determined by the interaction with its heterodimerization partner NF-YB. Most importantly, compartmentalization is regulated during the cell cycle of serum restimulated NIH3T3 cells, accumulating in the nucleus at the onset of S phase. These data point to the control of HFM heterodimerization as an important layer of NF-Y regulation during cell-cycle progression.