Substrates compete for access to CRBN. MM1S cells were transduced with lentivirus-expressing GFP and either a degron-deleted or K⁰ version of ZNF692. (A) Relative growth was assessed by means of flow cytometry after treatment with vehicle (solid lines) or 1 µM lenalidomide (dashed lines). Data are mean± SEM and are representative of 3 independent experiments. (B-D) Relative protein level as measured using the iMRM assay. Protein levels were normalized to a housekeeping factor and the level at the start of the experiment was normalized to 1. Curves represent the nonlinear regression. P values are from a sum of squares F test. Data are mean ± SEM (n = 3 replicates). ZNF692ΔZF, full-length ZNF692 protein with the CRBN-interacting zinc finger (amino acids 417-439) deleted; ZNF692 K⁰, full-length ZNF692 with every lysine changed to arginine.