Breast Cancer is a heterogeneous disease. To delineate heterogeneity and to exploit it as a principle for prognostic and therapeutic stratification still represents a major challenge in breast cancer research. Traditionally, in most cancers histo-logical typing and grading yields the basis for definition of subgroups. In breast cancer this stage of development has long been passed and novel principles are emerging. Gene expression profiling has enabled the definition of so-called 'intrinsic subtypes' of breast cancer. A decade ago the seminal study of Perou and co-workers [1] has paved the way to molecular classification. Although only a low number of cases were studied, the classification into luminal A and B type, basal type and HER2 type provided a useful frame for clinical decision-making. In fact, this classification was not completely new but reflected the expression of markers which are routinely assessed anyway in breast cancer by immunohistochem-istry. When molecularly defined intrinsic types are translated into categories to be assessed by immunohistochemistry two areas of incongruity can be identified. First, there is no validated tool to discriminate between luminal A and B type, although Ki-67 has been suggested by the St. Gallen consensus 2011 [2]. The intrinsic types and their immunohistochemical identification are discussed by Strehl and co-workers in this issue [3].
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