Exact solution of a two-type branching process: Models of tumor   progression
  An explicit solution for a general two-type birth-death branching process with one way mutation is presented. This continuous time process mimics the evolution of resistance to treatment, or the onset of an extra driver mutation during tumor progression. We obtain the exact generating function of the process at arbitrary times, and derive various large time scaling limits. In the simultaneous small mutation rate and large time scaling limit, the distribution of the mutant cells develops some atypical properties, including a power law tail and diverging average. 
Tibor Antal and P. L. Krapivsky
q-bio.PE cond-mat.stat-mech physics.bio-ph q-bio.CB