Seohyuk Lee

1 Introduction Breast cancer remains the most common cancer and the second leading cause of cancer-related deaths among women in the United States, despite sustained improvements in treatment, survival, and mortality over the past several decades [1]. Overexpression and amplification of human epidermal growth factor receptor 2 (HER2) are detected in approximately 15%–20% of all breast cancer cases and have historically been considered to be prognostic for poorer survival outcomes [2, 3].

1 Introduction Aging is a complex process and occurs at varying rates in individuals. Chronological age is the measure of time elapsed from an individual's birth to a given date, whereas biological age depends upon various biological and physiological factors such as genetics and comorbidities. Biomarkers of aging should differentiate between people of the same chronological age but different biological age [1-3].

CONFLICT OF INTEREST STATEMENT Elizabeth M. Cespedes Feliciano reports grants from the National Cancer Institute (R01CA240394). Justin C. Brown reports receiving grants from the National Institutes of Health and the Cancer Research United Kingdom and speaking fees from Novo Nordisk A/S and Nestlé Health Science for topics unrelated to this manuscript. All other authors have no conflicts of interest to disclose.