  A recently proposed single progenitor cell model for skin cell proliferation
[Clayton et al., Nature v446, 185 (2007)] is extended to incorporate
homeostasis as a fixed point of the dynamics. Unlimited cell proliferation in
such a model can be viewed as a paradigm for the onset of cancer. A novel way
in which this can arise is if the homeostatic fixed point becomes metastable,
so that the cell populations can escape from the homeostatic basin of
attraction by a large but rare stochastic fluctuation. Such an event can be
viewed as the final step in a multi-stage model of carcinogenesis. This offers
a possible explanation for the peculiar epidemiology of lung cancer in
ex-smokers.
