Transgenic animals that produce useful biological products can be created by the introduction of the portion of DNA which codes for a particular product to treat 

(a) Emphysema 

(b) Cystic fibrosis 

(c) Phenylketonuria

Transgenic animals that produce useful biological products can be created by the introduction of the portion of DNA (or genes) which codes for a particular product such as human protein (α-1-antitrypsin) used to treat emphysema. Similar attempts are being made for treatment of phenylketonuria (PKU) and cystic fibrosis.