SR-4835 (SR4835) is a novel highly selective dual inhibitor of CDK12 and CDK13 with anticancer activity. SR-4835 acts in synergy with DNA-damaging chemotherapy and PARP inhibitors and provokes triple-negative breast cancer (TNBC) cell death. inhibition or loss of CDK12/CDK13 triggers intronic polyadenylation site cleavage that suppresses the expression of core DNA damage response proteins. This provokes a 'BRCAness' phenotype that results in deficiencies in DNA damage repair, promoting synergy with DNA-damaging chemotherapy and PARP inhibitors.