Mammary development meets cancer genomics

Mammary epithelial cell development is thought to progress from undifferentiated stem cells into at least two differentiated cell types. A new study has now characterized some of these distinct developmental stages and links them to tumor subtypes previously defined by gene expression profiling (pages 907–913). Genomic studies of epithelial tumors have identified distinct subtypes with differences in survival and response to therapy, most notably for breast cancer1. Many researchers have speculated that these genomically defined tumor subtypes may represent transformation of stem cells with arrest at specific stages of development or, alternatively, direct transformation of various mature cell types.