The multistage carcinogenesis model. N0 denotes the number of normal stem cells. To get initiated, a normal cell accumulates n consecutive mutations, where ν denotes the mutation rate per cell per year for the gene in question. The number of cells having k mutations is noted I
, 1 ≤ k ≤ n. The fully initiated cells, I
, expand according to a birth-and-death process. These cells give rise to tumor cells T if a further event happens, and μ/(μ + β) can be interpreted as the probability of such a malignant transformation during a cell division.