By immunohistochemical and flow cytometric analyses, Vitiello and colleagues found that PDGFRA-mutant GIST contained more CD45+ and CD8+ cells, with a proportion of immune cells clustered around perivascular structures, a typical feature of adaptive immunity, confirming that PDGFRA-mutant GIST appear to be more immunologically active than KIT-mutant GIST with similar clinicopathologic features. The gene discussed is PDGFRA; the disease is gastrointestinal stromal tumor.