Somatic mutations in JAK1 have been identified in multiple tumor types including high-risk bladder cancer, endometrial, colorectal, stomach, and prostate carcinomas (17–19) supporting the idea that alterations in JAK1 signaling, whether through loss or gain of function, could play a role in the pathogenesis of some epithelial cancers. Here, JAK1 is linked to prostate carcinoma.