The effect of low HER2 expression on treatment outcomes in metastatic hormone receptor positive breast cancer patients treated with a combination of a CDK4/6 inhibitor and endocrine therapy: A multicentric retrospective study

•Combination therapy with CDK4/6 inhibitors and endocrine therapy achieves impressive objective response rates in metastatic hormone-positive breast cancer patients regardless of HER2 status.•Low HER2 expression did not significantly affect survival in hormone-positive metastatic breast cancer patients receiving CDK4/6 inhibitors and endocrine therapy.•The order of CDK4/6 inhibitor use and the presence of visceral metastasis were identified as independent factors influencing progression-free survival.