(a) Representative mechanisms of the formation of C-terminal cyclic aspartimide (cN) or glutarimide (cQ) and N-terminal pyroglutamate (pE) in vivo. (b) Structures of JQ1-uracil, JQ1-PEG-uracil, and JQ1-uridine. (c) Western blot of BRD4 after treatment with JQ1-uracil, JQ1-PEG-uracil, JQ1-uridine in HEK293T cells for 24 h. (d) Structures of JQ1-cQ, JQ1-cN, and JQ1-FpE. (e) Western blot of BRD4 after treatment with JQ1-cQ, JQ1-cN, JQ1-FpE in HEK293T cells for 4 h. All western blot data are representative of at least 2 independent replicates. For uncropped western blot images, see Supplementary Fig. 8.