Folded chromosomes from stationary phase or ammonia-starved yeast (Saccharomyces cerevisiae) cells can be isolated as compact structures, distinct and separable by sedimentation from the folded chromosomes of pre-replicative (G1) and post-replicative (G2) nuclei. Such cells are in a dormant or non-cycling (G0) stage. The folded genome from such cells is referred to as the g0 form and has a sedimentation velocity of about 1700S. Sedimentation analysis of mixed G0 and G1 and G2 lysates indicates that the g0 structure is not an artifactual breakdown product of the g1 or g2 structures. A comparison of the proteins from g0 versus g1 and g2 structures by gel electrophoresis has revealed differences in about 10--11 non-histone and perhaps 2 histone proteins. Entry into the G0 stage, and emergence into G1 after G0 arrest, are accompanied by an ordered transition from g2 to g1 to g0, and from g0 to g1 to g2 forms, respectively. Hence, entry into G0 and re-emergence from G0 can be considered as differentiative processes, not normally part of the cell cycle, and accompanied by specific changes in the tertiary organization of the genome.