To determine tumor-related factors that influence the response of metastatic melanoma tumors to treatment with the anti-CTLA-4 antibody ipilimumab, researchers have performed gene-profiling assays on biopsies collected from 45 patients before and after treatment with ipilimumab. In patients who responded to treatment, expression of immune-related genes increased, and expression of genes associated with cell proliferation and melanoma-specific antigens decreased. These data indicate that cell-mediated immune responses have a role in the activity of ipilimumab.