Compared to NF1-nonMUT, NF1-MUT pts had a shorter median rwTTNT on CDK4/6i regimens (4.2 vs. 12.4 mo, weighted log-rank p< 0.0001) and worse rwOS (15.8 vs 45.2 mo, weighted log-rank p=0.016)....Using multicenter clinical and real-world evidence, we demonstrate that NF1-MUT is associated with inferior duration and worse outcomes on CDK4/6i in MBC.