An intestinal crypt is shown with markers for cell cycle phases. The base of the crypt is on the left of the image. G0, early G1, and G2 cells are indicated with a blue dye for nuclei. The methodology used by Carroll et al. (2018) to monitor chromatin-bound Mcm2 identifies cells in early G1 and is indicated in red. Cycling cells are positive for EdU and are indicated in green. Phospho–histone H3, a marker of M phase, is visualized as whitish dots. The majority of cells in the transit-amplifying compartment were positive for EdU, indicating that they were actively cycling. At the base of the crypt, most of the cells are unlicensed and are likely in G0. There are some cells with chromatin-bound Mcm2 between the crypt base and the zone of actively proliferating EdU-positive cells. Image republished from Carroll et al. (2018) Fig. 3 A.