Western™ immunoassays revealed that HDAC6 levels remained largely unaffected by the treatment with the PROTACs. In contrast, HDAC1 levels showed significant degradation in the case of most compounds, particularly those with shorter spacers, except for 1j. The most prominent effect was observed for 1g, 1a, and 1j with 62.3, 61.9, and 52.8% degradation of HDAC1 at 1 µM. These three degraders were selected for further investigations. Degradation potency, target engagement, and selective HDAC1-3 degradation of 1a, 1g, and 1j. To further characterize the degradation profile of the three initial hits, we determined the maximal degradation (Dmax) and half-maximal degradation concentration (DC50) of 1a, 1g, and 1j for HDAC1 (Figure ). To this end, MM.1S cells were treated with the indicated compounds in increasing concentrations for 24 h. The most effective HDAC1 degrader from the initial screen, 1g, demonstrated an excellent Dmax of 85.2% and low DC50 of 257 nM. 1a exhibited a slightly lower Dmax in combination with a less favorable DC50, while 1j displayed a comparable DC50 combined with a lower Dmax compared to 1g. In addition, the degradation mediated by 1g was not only concentration dependent, but also time dependent (Figure , Supporting Information).