The present invention provides various compositions, including compounds of Formula (I) or (Ia), or a species thereof, and pharmaceutically acceptable salts, solvates (e.g., hydrates), stereoisomer, tautomers, isotopic and other specified forms thereof. Also provided are methods (or uses) and kits involving the compounds or pharmaceutically acceptable compositions containing them for treating or preventing a disease (e.g., a proliferative disease such as cancer) in a subject. Administration of a compound or pharmaceutical composition described herein is expected to inhibit cyclin-dependent kinase 7 (CDK7), and thereby, induce apoptosis in tumor cells in the subject.