BI-9321 trihydrochloride is a potent, selective and cellular active nuclear receptor-binding SET domain 3 (NSD3)-PWWP1 domain antagonist with a Kd value of 166 nM. BI-9321 trihydrochloride is inactive against NSD2-PWWP1 and NSD3-PWWP2. BI-9321 trihydrochloride specifically disrupts histone interactions of the NSD3-PWWP1 domain with an IC50 of 1.2 μM in U2OS cells.