Genetically modified mice are used extensively in research as models of human disease, the most common type is the knockout mouse. Transgenic mice generated to carry cloned oncogenes and knockout mice lacking tumors suppressing genes have provided good models for human cancer. Great care should be taken when deciding how to use genetically modified mice in researchIntroduction of foreign DNA into the mouse germ line is considered a major technical advancement in the fields of developmental biology and genetics. This technology now referred to as transgenic mouse technology has revolutionized virtually all fields of biology and provided new genetic approaches to model many human diseases in a whole animal context. Several hundreds of transgenic lines with expression of foreign genes specifically targeted to desired organelles/cells/tissues have been characterized. Further, the ability to spatio-temporally inactivate or activate gene expression in vivo using the “Cre-lox” technology has recently emerged as a powerful approach to understand various developmental processes including those relevant to molecular endocrinology