Figure 1.

Detailed history of the understanding of the heterogeneity of breast cancer. At first, breast cancer heterogeneity could only be assessed according to its clinical characteristics (stage, recurrence, metastasis). Technological advances provided further information on the pathologic heterogeneity in histologic classification and the expression of key molecules. During the past two decades, the rapid development of genetic analysis has enabled depiction of the heterogeneity of breast cancer on a genetic level.