In the next step, we investigated whether this target engagement translates into degradation of multiple HDAC isoforms. HDAC1, 2, 4, and 6 levels were determined by immunoblot analysis of MM.1S cell lysates and the CRBN-recruiting PROTAC A6 was used as control. The immunoblot analysis confirmed the automated Simple Western™ immunoassay results, as 1j showed most pronounced degradation of all tested HDAC isoforms (Figure ). In detail, the strongest maximal degradation (Dmax) was observed for HDAC1 (90%) by 1j, while the degradation of HDAC2, 4, and 6 ranged from 71-76%. The Dmax of compound 2, although slightly less efficient, was also the strongest for HDAC1 (74%) followed by HDAC2 (54%), while HDAC4 (40%) and HDAC6 (26%) were also degraded, but to a lesser extent (see Table , Supporting Information). In contrast, A6