Multistability in multiple state space dimensions. Subpopulations identified as homogeneous with respect to a single marker (i.e., CD11b) may be heterogeneous with respect to another, unmeasured marker (e.g., Marker X). The arrows show a hypothetical path of differentiation whereby cells first change marker X expression (a to b) and then CD11b expression (b to c). An isolated CD11bLow subpopulation thus may contain two (meta)stable states with respect to marker X (i.e. states a and b) that project into the same value with respect to CD11b.