Hypothetical models of G1 checkpoint activity.

<p>(A) The H phase begins Δ h <i>after</i> the M/G1 phase boundary and ends at the G1/S phase boundary. Cells can enter G0 at any point within the first Δ h of G1, and return at the same position, but not after. This model is consistent with Refs. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0192087#pone.0192087.ref022" target="_blank">22</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0192087#pone.0192087.ref027" target="_blank">27</a>]. (B) The H phase begins at the M/G1 phase boundary and ends Δ h <i>before</i> the G1/S phase boundary. Some cells enter into the cell cycle in G0 and reside there for some time before stochastically emerging into G1, which is the source of variability in this phase. This model is consistent with Refs. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0192087#pone.0192087.ref023" target="_blank">23</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0192087#pone.0192087.ref024" target="_blank">24</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0192087#pone.0192087.ref026" target="_blank">26</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0192087#pone.0192087.ref033" target="_blank">33</a>]. Note that Δ ≈ 3.5 h for A375 cells (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0192087#pone.0192087.g007" target="_blank">Fig 7</a>).