PREX1 and acute myeloid leukemia: Furthermore, in a recent gene essentiality profiling study28, a few genes were found to be synthetic lethal partners with oncogenic Ras genes, including the top-scoring gene PREX1, which encodes a guanine exchange factor (GEF) for RAC GTPases (e.g., RAC1) and works as an AML-specific activator of MAPK signaling.