Combining crystallization and mutational analysis, the foundations for understanding the interaction of CRBN with thalidomide and its analogues lenalidomide and pomalidomide were laid. These studies showed how the main pharmacophore structure, the conserved glutarimide ring, binds to CRBN (Figure ). It occupies a hydrophobic binding cavity between two CRBN β sheets and the carbonyls at C2 and C6 and amide at N1 form hydrogen bonds with CRBN. C3, C4 and C5 are in van der Waals contact with a tri-tryptophan hydrophobic pocket. Tyr386Ala and Trp388Ala amino acid substitution mutations altered the integrity of CRBN binding cavity and disrupted binding between CRBN and the glutarimide ring eliminating interaction of all 3 IMiDs to CRBN resulting in thalidomide and lenalidomide drug insensitivity in vivo. The C4-6 positions on the phthaloyl ring are solvent facing and differences in structural features at C4-6 influence both neosubstrate specificity and potency. For example, the amide at the C4 position in lenalidomide and pomalidomide specifies degradation of transcription factors IKZF1 and IKZF3 with a higher potency than thalidomide which lacks the C4 amide, while methyl and chloro substitutions at the C4 position increase degradation of IKZF1. Substitutions at C5 or C4,6 ablate IKZF1 degradation. This work has been serving as the foundation for molecular glue related SBDD. Figure . Scheme of thalidomide interactions with CRBN (G. gallus). Hydrophobic interactions are indicated as orange semicircles and hydrogen bonding is depicted as blue lines.