Fun Fact: Insulin was the first product ever sold that was genetically engineered in the sense that we use the term. E coli's genetics were carefully changed so that the bacteria would produce insulin identical to human insulin.
This was done to address the issues of cost, supply, and purity of animal insulins; unfortunately it was really an improvement only in that there is an unlimited supply of insulin you can get from bacteria, and in that fewer people have allergic reactions to human insulins. But I personally am happier to know that my insulin doesn't depend on killing any animals.
I actually learned that one in a class on genetically modified crops, a few months before I was diagnosed with diabetes. :-)