Question: Why are genetically modified organisms used?
About the article "DNA", section "Genetic engineering".

Candidate passages:
Genetically modified mammals are mammals that have been genetically engineered.
They are an important category of genetically modified organisms.
The majority of research involving genetically modified mammals involves mice with attempts to produce knockout animals in other mammalian species limited by the inability to derive and stably culture embryonic stem cells. 
A genetically modified organism (GMO) is an organism whose genetic material has been altered using genetic engineering techniques.
Organisms that have been genetically modified include micro-organisms such as bacteria and yeast, insects, plants, fish, and mammals.
GMOs are the source of genetically modified foods, and are also widely used in scientific research and to produce goods other than food.
The term GMO is very close to the technical legal term, 'living modified organism' defined in the Cartagena Protocol on Biosafety, which regulates international trade in living GMOs (specifically, "any living organism that possesses a novel combination of genetic material obtained through the use of modern biotechnology").
This article focuses on what organisms have been genetically engineered, and for what purposes.
The article on genetic engineering focuses on the history and methods of genetic engineering, and on applications of genetic engineering and of GMOs.
Both articles cover much of the same ground but with different organizations (sorted by organism in this article; sorted by application in the other).
There are separate articles on genetically modified crops, genetically modified food, regulation of the release of genetic modified organisms, and controversies. 
A genetically modified insect is an insect that has been genetically modified for various reasons such as agricultural production, oil production and pest control.
In biological research, transgenic fruit flies ("Drosophila melanogaster") are model organisms used to study the effects of genetic changes on development.
Fruit flies are often preferred over other animals due to their short life cycle, low maintenance requirements, and relatively simple genome compared to many vertebrates. 
Plants, animals or micro organisms that have changed through genetic engineering are termed genetically modified organisms or GMOs.
Bacteria were the first organisms to be genetically modified.
Plasmid DNA containing new genes can be inserted into the bacterial cell and the bacteria will then express those genes.
These genes can code for medicines or enzymes that process food and other substrates.
Plants have been modified for insect protection, herbicide resistance, virus resistance, enhanced nutrition, tolerance to environmental pressures and the production of edible vaccines.
Most commercialised GMO's are insect resistant and/or herbicide tolerant crop plants.
Genetically modified animals have been used for research, model animals and the production of agricultural or pharmaceutical products.
They include animals with genes knocked out, increased susceptibility to disease, hormones for extra growth and the ability to express proteins in their milk. 
This is a timeline of noteworthy genetically modified organisms created in labs, including genetically modified foods and other transgenic species and products (such as Glofish).