MiST1 was a single centre, open label single arm phase IIa design with prospective molecular stratification; cytoplasmic/negative BAP1 or BRCA1 deficient MM was deemed eligible. Treatment was...rucaparib...Molecular eligibility was 89% for BAP1 alone, 50% BRCA1 alone, and 39% BAP1+BRCA1....MiST1 using the PARP inhibitor R met its primary endpoint of disease control rate at 12 weeks, showing promising efficacy with manageable toxicity.