Bt cotton contains a toxin gene (cry gene) isolated from the bacterium Bacillus thuringiensis. This gene codes for specific crystal proteins (Cry proteins) that are toxic to certain insect pests such as lepidopterans (tobacco budworm, armyworm), coleopterans (beetles), and dipterans (flies, mosquitoes). The introduction of this gene makes the plant resistant to these insect pests, reducing the need for chemical pesticides.