Human proteins are in great demand for the treatment of a variety of diseases.
Whereas some can be purified from blood, this is expensive and runs the risk of contamination by AIDS or hepatitis C. Proteins can be produced in human cell culture but costs are very high and output small. Much larger quantities can be produced in bacteria or yeast but the proteins produced can be difficult to purify and they lack the appropriate post-translational modifications that are needed for efficacy in vivo . By contrast, human proteins that have appropriate post-translational modifications can be produced in the milk of transgenic sheep, goats and cattle. Output can be as high as 40 g per litre of milk and costs are relatively low. In the US, Genzyme Transgenics have focussed on goats and their lead product, tissue plasminogen activator, is also in clinical trials.