A number of studies have recently demonstrated that super-enhancers, which are large cluster of enhancers typically marked by a high level of acetylation of histone H3 lysine 27 and mediator bindings, are frequently associated with genes that control and define cell identity during normal development. Super-enhancers are also often enriched at cancer genes in various malignancies. The identification of such enhancers would pinpoint critical factors that directly contribute to pathogenesis. In this study, we performed enhancer profiling using primary leukemia samples from adult T-cell leukemia/lymphoma (ATL), which is a genetically heterogeneous intractable cancer. Super-enhancers were enriched at genes involved in the T-cell activation pathway, including , and in both ATL and normal mature T cells, which reflected the origin of the leukemic cells. Super-enhancers were found at several known cancer gene loci, including , and , in multiple ATL samples, but not in normal mature T cells, which implicated those genes in ATL pathogenesis. A small-molecule CDK7 inhibitor, THZ1, efficiently inhibited cell growth, induced apoptosis, and downregulated the expression of super-enhancer-associated genes in ATL cells. Furthermore, enhancer profiling combined with gene expression analysis identified a previously uncharacterized gene, , that was associated with super-enhancers in all ATL samples, but not in normal T cells. Knockdown of induced apoptosis in ATL cell lines, whereas overexpression of this gene promoted cell growth. Our study provides a novel strategy for identifying critical cancer genes.