Here n is the total number of tumors sequenced, k is the number of tumors with mutations in the region of interest, Fm is the set of all subsets of k integers selected from {1,2,…,n}, A is a subset of Fm, Ac is the complement of set A, pi is the probability of mutation in tumor i, and pj is the probability of mutation in tumor j. The Poisson binomial probability is calculated using an efficient and accurate normal approximation in the “poibin” R package.