FGFR4 and neoplasm: cfDNA profiling of the baseline enrollment sample prior to palbociclib commencement showed low tumor fraction (HR = 2.28; P = 9.9 × 10−6); higher short/long fragment length ratios (HR = 1.19; P = 0.049) and cfDNA variants in FGFR4 (HR = 3.65; P = 0.012) were prognostic and associated with inferior PFS.