Scientists have discovered that people with cancers containing genetic mutations JAK1 or JAK2, which are known to prevent tumors from recognizing or receiving signals from T cells to stop growing, will have little or no benefit from the immunotherapy drug pembrolizumab. This early-stage research has allowed them to determine for the first time why some people with advanced melanoma or advanced colon cancer will not respond to pembrolizumab, an anti-PD-1 treatment.