BRK inhibitor P21d hydrochloride is a highly potent inhibitor of breast tumor kinase (BRK PTK6), displaying an IC50 of 30 nM. Additionally, it effectively suppresses p-SAM68 with an IC50 value of 52 nM. This compound, BRK inhibitor P21d hydrochloride, serves as a valuable tool for evaluating the efficacy of BRK inhibitors in xenograft breast tumor models, enabling the assessment of their in vivo activity.