Background: Breast cancer incidence is increasing rapidly among many Asian populations primarily due to the adaptation of a Westernized lifestyle. However, Asian women seem to have a distinctive profile of breast cancer that differs from that seen in Western populations, such as earlier age at onset, more advanced stage, and higher frequency of ER-negative tumors. The disproportionally high rates among younger women were also seen among U.S.-born Asian women, such as Chinese, Filipina, Indian, and Pakistani women. These findings suggest that Asian breast cancer may have a distinct etiology that is not fully explained by adapting a westernized lifestyle. Recent molecular studies have suggested that breast cancers comprise several biologically and clinically distinct tumor subtypes. Epidemiologic evidence suggests that associations with breast cancer risk factors may vary by tumor subtype. However, most previous studies were conducted in Western populations; large, population-based epidemiologic studies characterizing molecular subtypes of breast cancer in Asian women are limited. In this study, we would like to characterize the molecular profile of breast cancer among Chinese women in Hong Kong and associate the molecular features with clinical and etiologic factors. An improved understanding of the inter-relationships among age, breast cancer subtypes, and race/ethnicity is critical for the development of targeted prevention and the best and most efficacious therapy.