Interestingly, all hits induced HDAC1 degradation without affecting HDAC6 levels, despite being equipped with an unselective HDAC recruiter. This raises the questions of whether target binding is altered by the incorporation of the vorinostat-like ligand into the FEM1B-engaging PROTACs and which other HDAC isoforms might be degraded. To clarify the first question, we conducted in vitro HDAC enzyme inhibition assays, which are summarized in Table . The assays revealed that the lowest half maximal inhibitory concentration (IC50) values were observed for HDAC6 inhibition with IC50 values ranging from 0.004 to 0.009 µM, while the IC50 values for HDAC1 and recruiting PROTACs outperformed the control inhibitors vorinostat and ricolinostat in inhibiting HDAC1, 2, and 6, while for all compounds under study the inhibition of HDAC4 was weak. Since all selected PROTACs demonstrated engagement of multiple HDAC isoforms, the degradation profile was investigated in more detail. Immunoblot analysis was conducted to examine degradation of all HDAC isoforms from class I (HDAC1, 2, 3 and 8) and representative isoforms for class IIa (HDAC4) and IIb (HDAC6). Remarkably, the FEM1B-based PROTACs induced strong and selective degradation of HDAC1-3 while all other investigated isoforms remained unaffected (Figure ). This degradation pattern stands in pronounced contrast to our previously published CRBN-based PROTAC A6, which selectively degraded HDAC6. These findings confirm that FEM1B recruitment is not only suitable for HDAC degradation, but also offers selectivity for HDAC1-3. For instance, comparing A6 and 1j -both of which share the same HDAC ligand and spacer but differ in the E3 ligase warhead -revealed a switch in selectivity from HDAC6 degradation to HDAC1-3 degradation. This highlights the potential for fine-tuning PROTAC selectivity by altering the recruited E3 ligase.