Label retention assay. All cells within a living tissue (white circles, left) are labeled during the pulse period (blue cells). The labeling method can be either chemical, such as 5-bromo-2'-deoxyuridine (BrdU), or genetic, such as histone 2B-green fluorescent protein (H2B-GFP). During the chase period, labeling stops and the cells dilute the dye according to the their proliferation rate. Fast proliferating cells (top row) dilute the dye faster than slower proliferating cells (middle row). Quiescent cells retain the dye, which can still be detected at the end of the chase period.