“…We used the proliferating cell nuclear antigen (PCNA)-mCherry fluorescent reporter to quantify, for each cell, the duration of G1, S, G2, and M, and, implicitly, the entire cell cycle duration (Chao et al, 2017; Fig 1A, Appendix Fig S1). It has been firmly established in previous studies that, during S phase, PCNA is loaded at DNA replication forks and forms foci in well-described punctate patterns (Madsen & Celis, 1985;Kennedy et al, 2000;Leonhardt, 2000;Wilson et al, 2016;Chao et al, 2017). PCNA localization is precisely correlated with DNA replication and thus is a bona fide marker of S phase (Madsen & Celis, 1985;Leonhardt, 2000;Burgess et al, 2012;Wilson et al, 2016;Chao et al, 2017;Zerjatke et al, 2017).…”