DC-peptoid-1 is a specific binding agent and cross-linker for the phosphorylated Brd4 PDID region, exhibiting a dissociation constant (Kd) of approximately 50-100 μM for human targets. It selectively binds only phosphorylated PDID, without recognizing the non-phosphorylated form or other regions like Brd4 598-785. DC-peptoid-1 efficiently cross-links with target proteins in both solution and cell lysates and can successfully capture phosphorylated PDID from complex systems through immobilization. It does not bind to non-phosphorylated proteins from bacteria or other nonspecific phosphoproteins. This compound has the potential to serve as an alternative to phosphoprotein-specific antibodies in applications such as immunoaffinity purification.