Recent reports detailing the expression profiles of primary breast cancer have pointed to the utility of this approach in defining subclasses with distinct molecular configurations and clinical behaviour. Some of the subclasses can be predicted by current molecular tests: estrogen receptor status, p53 staining, and HER-2 overexpression. Others, however, are novel subgroups and may represent distinct cellular types. The results from two recent studies suggest common principles of classification by expression profiling. These principles are examined and the impact of these results on understanding the biology and the clinical behaviour of breast tumors is explored.