(1) Fluorescent proteins are found in bioluminescent sea creatures, such as jellyfish. (2) In 1969, the green fluorescent protein was discovered and, since then, prolific research has resulted in the discovery and development of many new fluorescent proteins. (3) Fluorescent proteins can be expressed in cells, making them easily visible under a microscope. (4) In fact, the use of fluorescent proteins revolutionized bio-imaging because the inherently natural nature meant that they could be incorporated into living cells with no deadly toxic effects. (5) Numerous other scientific and medical applications also exploit the unique properties of fluorescent proteins, which is why they were the subject of the 2008 Nobel Prize for Chemistry. (6) However, the science behind fluorescent proteins is still not fully understood, and there are many more avenues for exploration.
What is implied by the fourth sentence of the passage?