Thalidomide and its derivatives, lenalidomide and pomalidomide, are immune modulatory drugs (IMiDs) used in the treatment of haematologic malignancies. IMiDs bind CRBN, the substrate receptor of the CUL4–RBX1–DDB1–CRBN (also known as CRL4CRBN) E3 ubiquitin ligase, and inhibit ubiquitination of endogenous CRL4CRBN substrates. Unexpectedly, IMiDs also repurpose the ligase to target new proteins for degradation. Lenalidomide induces degradation of the lymphoid transcription factors Ikaros and...