Small cell lung cancers, triple negative breast cancer, and T‐cell acute lymphoblastic leukemias are each aggressive cancers with high mortality rates; notably, each is sensitive to THZ1, a covalent inhibitor of CDK7.198, 199, 200, 201 As expected, CDK7 inhibition by THZ1 is broadly cytotoxic,200 and although THZ1 inhibits other kinases, it represents a powerful means to assess potential roles for CDK7 kinase activity in cancer cell proliferation. Here, CDK7 is linked to cancer.