  In order to analyze data from cancer genome sequencing projects, we need to
be able to distinguish causative, or "driver," mutations from "passenger"
mutations that have no selective effect. Toward this end, we prove results
concerning the frequency of neutural mutations in exponentially growing
multitype branching processes that have been widely used in cancer modeling.
Our results yield a simple new population genetics result for the site
frequency spectrum of a sample from an exponentially growing population.
