Twenty years of transgenic animals: Are some inventions SO important as to NOT be entitled to full patent protection?

Transgenic animals have been known for 30 years but the first transgenic mammal patent dates from 1988. This first patent, for the Oncomouse which has been created to be susceptible to developing tumors, has generated large revenues for its licensee, DuPont. Equivalent patents were only granted in Canada and Europe after litigation and amendment of the patent. DuPont has strictly enforced its patent rights but this has led to debate over whether it is impeding research.