To find this clue, you would have discovered the lac operon operator, which is regulatory region that allows for some microorganisms like E. coli to process lactose as a energy source.
Let's have a look!
The lac operon was discovered nearly 100 years ago in E. coli bacteria and it unveiled for the first time the sophistication of genetic circuits in cells. This discovery was made by François Jacob and Jacques Monod in the 1960's after Monod began looking at the carbon sources E. coli could use for energy.
In the initial phases of this experiment, Monod discovered that if E.coli was given both lactose and glucose in the agar medium it grew on, the bacteria would always use the glucose first until it had to start using the lactose. The E. coli would not even begin synthesizing (creating) the proteins required to use lactose as an energy source until all the glucose was used up. This lead to the discovery of the regulatory region, the operator, that allowed the E. coli to choose when to synthesize lactose.
The lac operon operator is only activated when there is both a presence of lactose and an absence of glucose. When both conditions are met, the bacteria synthesizes the lactose as the primary carbon/energy source. This explained why the E. coli wouldn't start processing the lactose until the glucose had run out.
The lac operon then became the first genetic regulatory mechanism to be understood and it became the foundation for all other prokaryotic bacteria gene regulation. Prokaryotes are organisms whose cells lack a nucleus - animals, humans, plants, fungi and yeast are not prokaryotes, they are eukaryotes so the operon does not work for them.
If you are keen to learn more on the lac operon and you have the Zero to Genetic Engineering Hero book have a look at Chapter 7! The Induce-it kit, which lets you chemically turn on the color gene in bacteria, is based on the lac operon.