The present invention provides novel compounds of Formula (I) and Formula (II) and pharmaceutically acceptable salts, solvates, hydrates, tautomers, stereoisomers, isotopically labeled derivatives, and compositions thereof. Also provided are methods and kits involving the compounds or compositions for treating or preventing proliferative diseases (e.g., cancers (e.g., leukemia, melanoma, multiple myeloma), benign neoplasms, angiogenesis, inflammatory diseases, autoinflammatory diseases, and autoimmune diseases) in a subject. Treatment of a subject with a proliferative disease using a compound or composition of the invention may inhibit the aberrant activity of cyclin-dependent kinase 7 (CDK7), and therefore, induce cellular apoptosis and/or inhibit transcription in the subject.