Cell-cycle age density
= Cell-cycle age density
{title2=$n_i(\tau,t)$}
A <cell-cycle age density> counts <biological cells> per unit time spent in one specified phase of the <cell cycle>. Its integral over phase age is the population in that phase. Phase age resets on entry into the next phase, so it differs from chronological age since birth. A phase-exit <hazard function> supplies loss from the phase and incoming <flux> to the next phase.