Phosphatidylinositol 3,4,5-trisphosphate-dependent Rac exchanger 1 (P-Rex1) is a Rac guanine nucleotide exchange factor (RacGEF) that is upregulated in numerous adult cancers, including breast cancer, prostate cancer, melanoma, and liver cancer (11–15). The gene discussed is PREX1; the disease is cancer.