Myoglobin occurs in many species, including sperm whales from which the crystals here (enlarged 40 times) were obtained.

Myoglobin is an oxygen-binding protein found in the muscle cells of animals. It functions as an oxygen storage unit, providing oxygen to the muscles. Diving marine mammals, such as seals and whales, are able to remain submerged for long periods because they have greater amounts of myoglobin in their muscles than other animals do. There is a close chemical similarity between myoglobin and hemoglobin, the oxygen-binding protein of red blood cells. In 1962, John Kendrew shared the Nobel Prize in Chemistry for his construction of a three-dimensional crystalline model of sperm whale myoglobin.