Do distinct subpopulations signify modes of behaviour in a noisy single cell?

Abstract Population-level distributions of fluorescence or molecule counts are often taken to reflect the behaviours of individual cells within that population. In this conceptual article, I argue that counting subpopulations can be a misleading proxy for identifying the number of behavioural modes accessible to individual cells within a system. I show that definitions of behavioural modes based on deterministic modelling can fail when fluctuations in a system’s state—or noise—become significant