Finally, to complement our previous assessment of BRD4 degradation by Western blots, we investigated photoinduced BRD4 knockdown in real time using live cell fluorescence imaging. For this purpose, a GFP-BRD4 expression construct was generated for transient expression in HEK293 cells. Prior to transfection, we also confirmed that PROTAC 3 degraded BRD4 in HEK293 cells when irradiated (Supporting Information, Figure ). Similar to the data gathered from HeLa cells, 1 μM compound 2 delivered significant GFP-BRD4 knockdown within 2 h as shown by the rapid decrease in fluorescence as compared to the DMSO control (Figure , 6D and 6E). On the other hand, PROTAC 3 had no effect on GFP-BRD4 levels until it was uncaged to deliver an active degrader that removed GFP-tagged BRD4 within the same length of time as 2 (Figure , 6B and 6E). A comparable degradation profile was obtained with 5 μM PROTAC 3 following irradiation (Supporting information, Figure ).