Final answer:
Pigs were the source of insulin used to treat diabetic patients before recombinant DNA techniques enabled the production of human insulin in bacteria.
Explanation:
Before recombinant DNA techniques were used to express the human hormone insulin in E. coli, the source of insulin used to treat diabetic patients was pigs. Pig insulin was utilized; however, it caused allergic reactions in humans due to differences in the gene product. Recombinant DNA technology revolutionized insulin production, enabling the creation of human insulin in bacteria like E. coli, which is now the preferred choice for diabetes treatment.
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