excellent models of human disease, which is one of the reasons that 90% of all animals used in U.S. research today are rats and mice.


Newborn “green mouse”

Genetically modified animals (sometimes called transgenics) are developed by altering an embryo’s genes, either by treatment with chemicals or by adding or removing a gene. Most genetically modified animals are mice that are developed to mimic a human disease. For example, one type of genetically modified animal has the gene for a protein called pro-opiomelanocortin (POMC) “knocked out” or made nonfunctional. POMC is converted by the body into hormones that influence pigmentation, food intake, and fat storage. People with alterations in the POMC gene become severely obese as infants and have red hair. POMC knock-out mice are severely obese and have yellow rather than brown fur, mimicking the human disease. POMC knock-out mice are being studied to understand how the body controls hunger and metabolism. They are also used to test new drugs to treat obesity.

Genetically modified animals can be useful even if they are not created to mimic a disease. Take for instance the “green mouse.” Green mice have a jellyfish gene called “green fluorescent protein” inserted into their genomes. This jellyfish gene encodes a protein that glows green under ultraviolet (UV) light. Therefore, when green mice are exposed to UV light, every single cell in their body glows green. Scientists use these mice to study a wide variety of immune diseases, even though these mice are perfectly normal. By studying green mice, scientists have studied how immune cells from the mother are passed to infants through breastfeeding, how different immune cells interact, and how fetal immune cells migrate out of the womb and into the mother’s organs, which may cause some autoimmune diseases.

GENOME—All of the genetic material or DNA of an organism.

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