Table 1.

Some types of physiological variables now quantifiable in single cells

Type of variable	Example variable	Example quantification methods	Example state terms
Continuous variables defining classical states	pH; glutathione redox	Signal from fluorescent protein derivatives engineered to change emission intensity or emission spectrum on change in measured condition	‘Acidic’; ‘highly reducing’
Discrete or integer variables	Double‐stranded DNA breaks; number of specific mRNAs	Signal from fluorescent protein derivative engineered to form patches on double‐stranded DNA ends (DSEs); signal from fluorescent protein derivative that binds sites in 3′ region of mRNAs transcribed from engineered genes	‘Possessing double‐stranded breaks’; ‘numerous copies of a particular mRNA’
Continuous variables defining other nonclassical states	‘Growth‐factor‐dependent‐mTORC1 status’	Signal from fluorescent protein derivative that changes FRET signal on phosphorylation by mTORC1	‘Highly growth factor stimulated’, ‘rapamycin inhibited’
Continuous variables defining ‘systems level’ states	G (gene expression capacity)	Cell's position on a correlated variation axis made by plotting the outputs of two different promoters expressing two different fluorescent proteins	‘High gene expression capacity’