This promoter only switches on in the presence on Cd2+ ions. Below is an image of bacteria carrying the promoter as well as the red fluorescent protein gene in varying metal ion solutions. As you can see, even other heavy metals with 2+ charge can’t activate the promoter.
Cadmium serves no biological purpose, is very toxic and is classed as a carcinogen. Inhaling or ingesting any significant amount of cadmium can cause immediate poisoning and damage to the liver and kidneys.
Symptoms of cadmium poisoning can include softening of bones and loss of bone mineral density and in extreme cases this can result in body weight being enough to cause fractures. The kidney’s also permanently lose their ability to remove acids from the blood which results in an increase in phosphate levels causing muscle weakness. You can learn more about the health effects of cadmium here.
Below is a graph showing how effective the promoter is at different concentrations of Cd2+. It shows that the promoter requires a minimum threshold between 10-8mol/L and 3*10-8mol/L.