6-4.1 Introduction:

As discussed earlier, transgenic animals are genetically modified by stable incorporation of foreign DNA into its genome by artificial gene transfer technology in order to introduce, silence or delete specific phenotypic characteristic. Transgenic animals can be used to express recombinant proteins in milk, urine, blood, sperm, or eggs, or even to grow organs for xenotransplantation. “Biopharming” in animals encompass the production of biologically active recombinant proteins using transgenic animals as bioreactor.

Although production of recombinant human therapeutic proteins in microbial bioreactors is extremely cost-efficient, it suffers from many limitations like:

1. Prokaryotic cells lack post translational modification machinery (glycosylation, phosphorylation etc) essential for proper folding and functioning of complex eukaryotic proteins.  
   
   
2. The lack of post-translational modifications in prokaryotic system can be overcome by replacing them with large-scale animal cell cultures. But the long generation time and the requirement for rich culture media make animal cell bioreactors more expensive.
   
   
3. In both the above cases, the upstream production operations are relatively inexpensive but the downstream processing of the final product is sometimes extremely complex and costly.

These drawbacks can be overcome by using transgenic animals for production of human or veterinary pharmaceuticals. The use of targeted gene transfer for the expression of the transgene in the mammary gland of large farm animals like cow, pigs, goats and sheep offers advantages like easy isolation of the product secreted into the milk and low cost-large scale production.