Background: CDK4/6i plus ET is standard of care first-line (1L) therapy for hormone receptor positive (HR+), HER2- aBC. While molecular analyses from trials and pts have identified several mechanisms of resistance to CDK4/6i plus ET, currently, no biomarkers (outside of hormone status) are used for therapy selection. We utilized RW data to assess impact of putative baseline CDK4/6i+ET resistance alts (CDK4/6i+ET-R) on clinical outcomes among pts with aBC treated with CDK4/6i. Methods: Pts with aBC were identified via the Guardant INFORM database; those with a ctDNA test within 90 days prior to CDK4/6i start were included in this analysis. Based on previous studies, oncogenic or likely oncogenic alterations (as characterized by OncoKB) in the following genes were considered CDK4/6i+ET-R alts: RB1, PTEN, AKT1, CCNE1, ERBB2, FGFR1, FGFR2, and KRAS. Pts with >1 CDK4/6i+ET-R alt were compared to pts without any CDK4/6i-R alts. Kaplan-Meier plots, log-rank tests (LR) and multivariate Cox proportional hazards models (HR) adjusted (adj) for age, sex, year of ctDNA test and line of therapy were used to assess differences in time to next treatment (TTNT) and overall survival (OS). Results: 1075 pts with aBC treated with CDK4/6i (27% receiving 1L treatment) met inclusion criteria, 293 of whom (27%) had >1 alt in a CDK4/6i+ET-R gene. Pts with CDK4/6+ETi-R alts had significantly shorter TTNT (8.7 mos vs 14.2 mos, LR p=0.001; adj HR 1.36, 95%CI 1.11-1.66, adj p=0.003) and OS (28.3 mos vs 60.9 mos, LR p=<0.0001; adj HR 2.19, 95%CI 1.69-2.82, adj p=<0.0001) compared to those without. This divergence was more pronounced for patients receiving 1L CDK4/6i (TTNT: 10.7 mos vs 24.6 mos, LR p=0.006, adj HR 1.84, 95%CI 1.18-2.87, adj p=0.007; OS: 23.8 mos vs 67.3 mos, LR p<0.0001, adj HR 3.27, 95%CI 1.89-5.62, adj p<0.0001). Individual gene contributions towards less favorable TTNT are shown. Conclusions: We demonstrate via RW data that patients with >1 pre-treatment ctDNA alt in a curated list of CDK4/6i+ET-R genes have significantly worse outcomes on CDK4/6i plus ET treatment, a difference more pronounced among patients receiving 1L therapy. This data is among the first successful efforts to validate a clinically useful multi-gene clinical biomarker that has the potential to predict CDK4/6i plus ET resistance. Further exploration of this signature’s prognostic and predictive utility for CDK4/6i plus ET use is ongoing to better refine personalized treatment selection in HR+/HER2- aBC.