Whole genome sequencing identified 2 subtypes of FL with differing risks of histologic transformation. Aberrant somatic hypermutation (aSHM) induced by AID was associated with increased risk of transformation and increased number of mutations shared with GCB DLCBL. This subset was named DLBCL-like FL (dFL). Conversely, the cohort with lower risk of transformation showed less aSHM and fewer overall mutations, and enrichment for missense mutations of CREBBP, ATP6V1BP2, ATP6AP1, and RRAGC. This subset was named constrained FL (cFL).