15 April 1922
Insulin is a peptide hormone used to treat diabetes for patients that would otherwise die. Before its discovery, people with diabetes had had a very short life span. It was first discovered by Frederick Banting and J.J.R Macleod in 1922. The discovery meant that Diabetes sufferers could now take a daily treatment that would help them lead a normal life. Banting was working with J.J.R. Macleod and Charles Best in the University of Toronto in Canada. Both Banting and Macleod were awarded the Nobel Peace Prize for their work, while Best was excluded, a decision the Nobel Peace Prize committee subsequently regretted.
References

The Nobel Prize, The Discovery of Insulin, from The Nobel Prize, [online], Available at www.nobelprize.org [Accessed: 05 April 2013]|Britannica, Who Really Discovered Insulin?, from Encyclopaedia Britannica, [online], Available at www.britannica.com [Accessed: 05 April 2013]