Figure 2.

(a) The five possible transitions of a single cell at an initial stage (white) and their probabilities a_mn. Dividing cells can produce daughters at a more differentiated or mutated stage (red). Since these are the only possible steps, a₀₀ + a₀₁ + a₁₁ + a₂₀ + a₀₂ = 1. (b) When inter-transition times are exponentially distributed, the rates of each process can be defined in terms of the branching rate and the branching probabilities $a_{mn}^{(k)}$ at each node k.