Transgenic mouse technology provides the capabilities to produce transgenic mice by microinjection of foreign DNA into fertilized mouse embryos. Transgenic mice allow investigators to study gene expression and function in the context of the whole organism and during dynamic processes such as embryonic development, puberty, sexual reproduction, tissue regeneration and repair, response to environmental factors, learning, and aging. In addition, the effect of modifying mutations and genetic background on a disease process can be investigated. In the study of diseases as complex as cancer, genetically engineered mice can provide important and novel insights.