ESR1 and cancer: 9p21 deletion was linked to adverse tumor features, including high-grade (p < 0.0001) and nodal positive cancers (p = 0.0063), high cell proliferation (p < 0.0001), negative hormone receptor (ER/PR) status (p ≤ 0.0006), and HER2 amplification (p = 0.0078).