ERBB2 and breast cancer: To address the limited understanding of kataegis in early-stage primary breast cancer, our study aimed to comprehensively describe, characterize, and analyze the association of kataegis with clinicopathological and molecular factors, transcriptional patterns, and patient outcomes with a focus on established clinical and molecular subgroups defined by ER, PR, HER2, PAM50, and homologous recombination deficiency (HRD) status.