53BP1-binding protein 1 (53BP1) engages with dimethylated lysine 20 on histone 4 (H4K20me2) through its tandem tudor domains within a homodimer configuration, crucial for the DNA damage response. UNC2170, a micromolar 53BP1 ligand, selectively interacts with this site, demonstrating at least 17-fold preference for 53BP1 over similar proteins. This interaction occurs in a pocket formed by the 53BP1's tudor domains. Moreover, UNC2170 acts as an antagonist to 53BP1 in cellular lysates, effectively inhibiting class switch recombination, a process dependent on the functional 53BP1 tudor domain, thus confirming its cellular activity.