Breast cancer is a significant world health problem, as the second most commonly diagnosed cancer in the world [1]. It is characterized by heterogeneity in associated risk factors, histological types, and biological and clinical tumor behavior [2]. There is marked worldwide variation in the incidence pattern of breast cancer. In most high income countries, the incidence per 100000 population is high but the 5-year survival rate is also high (US 99.4 and 81%, respectively, western Europe 84.6 and 74%, respectively) [1]. Japan has a low incidence (32.7 per 100 000 population) and a high survival rate (75%) [1]. By contrast, in many middle- to low-income countries, the incidence per 100 000 population is low but the survival rate is also low (India 21.8 and 46%, respectively, South America 25.9 and 67%, respectively, sub-Saharan Africa 19–33 and 32%, respectively) [1]. The theory that differential exposure to risk factors and dissimilar distribution of the biological traits predisposing to breast cancer exist among different communities represents a logical explanation for these observations. Indeed, the etiology of high-risk, early-onset breast cancers, which are more prevalent in middle- to low-income countries, probably differs from that of low-risk, late-onset breast cancers, which are more prevalent in high-income countries [3]. Differences in environmental factors are thought to play an important role in this variation, as demonstrated by the finding that immigrants from areas of low incidence into areas of high incidence tend to acquire the breast cancer rate of the host population within one to two generations [4]. However, this does not explain all of the differences seen among different ethnic groups, which can exist within the same population.