PBRM1 and cancer: While some CRFs (e.g., MLL3, MLL2) appear mutated in several cancer types, others are very specific to one or a few tumor types (e.g., ATRX, PBRM1), as apparent from differences in the distribution of the overrepresentation of CRF driver mutations (with respect to the expected value) in each tumor sample across all the cohorts analyzed, a metric which we call the CRF-to-driver index, or CDI (Methods).