ERBB2 and breast cancer: The collective analysis revealed that while CDK4/6 inhibitor combined with endocrine therapy effectively suppressed the proliferation of both HR+/HER2-0 and HR+/HER2-low breast cancer cells, the inhibitory effect on HR+/HER2-low breast cancer was notably attenuated compared with the HR+/HER2-0 subtype (P < 0.01) (Fig. 2A,B).