Pomalidomide is an E3 ligase recruiter exploited by PROTACs to degrade target proteins, but its application is hampered by the off-target degradation of other vital endogenous zinc finger (ZF) proteins. Now, the off-target ZF binding of pomalidomide-based PROTACs is evaluated by a high-throughput imaging screening platform, and minimization of off-target degradation as well as enhanced potency are achieved through selective functionalization at the C5 position of the phthalimide ring.