BRD4 degrader AT1 is a highly selective Brd4 degrader, with a Kd of 44 nM, for Brd4 in cells. BRD4 degrader AT1 also shows Kds of 38.8 nM for a mutant Brd4 (QVK), 111 ± 14 nM for Brd2, 94 ± 9 nM for Brd2, 35 ± 3 nM for Brd3, 39 ± 8 nM for Brd3, 75 ± 23 nM for Brd4 and 35 ± 4 nM for a Brd2 mutant (KEA). BRD4 degrader AT1 (1-3 μM) causes remarkable Brd4-selective depletion and has no or little activity against Brd2 and Brd3.