Insect-resistant cotton plants have been genetically engineered by inserting a gene from a bacterium. Specifically, the gene from the bacterium Bacillus thuringiensis (Bt) is commonly used. This bacterium produces a protein called Bt toxin, which is toxic to certain insects. By inserting the Bt gene into the cotton plants, they can produce the Bt toxin themselves, making them resistant to insect pests such as bollworms and budworms. The Bt toxin kills the insects when they ingest the cotton plant, providing a built-in form of insect resistance.