Abstract
A transgenic mouse line expressing Fucci (fluorescent ubiquitination-based cell-cycle indicator) probes allows us to monitor the cell cycle in the hematopoietic system. Two populations with high and low intensities of Fucci signals for Cdt1(30/120) accumulation were identified by FACS analysis, and these correspond to quiescent G0 and cycling G1 cells, respectively. We observed the transition of immune cells between quiescent and proliferative phases in lymphoid organs during differentiation and immune responses.