Pts with alterations in FRS2 and PRKCA (treatment interaction P< 0.05) as well as MDM2, ERBB2, AKT1, and BRCA1/2 (P> 0.05 but considered actionable) had a trend for increased PFS benefit of RIB vs PBO (Table)….Results of this pooled analysis of the ML-2, 3, and 7 trials, the largest biomarker analysis of any CDK4/6 inhibitor in ABC, revealed several potential biomarkers of response (FRS2, MDM2, PRKCA, ERBB2, AKT1, and BRCA1/2) or resistance (CHD4, BCL11B, ATM, or CDKN2A/2B/2C) to RIB.