Many "man made" toxins are released into the environment where they accumulate in the plants and animals present, sometime causing great harm. Some animals, such as Mytilus californianus [mussel], are resistant to some of these toxins. They use a transporter protein, called MDR [multi-drug-resistant], to expell the toxin once it has entered the cell.

Why do mussels have a protein in their gills that provides a defense to the fairly recently encountered human toxins [thinking on an evolutionary time scale]? Well, plants and animals also produce toxins to avoid being eaten. The mussel has an advantage of being able to eat and live in an additonal environments that more sensitive creatures, like the sea urchin Stronglyocentrotus purpuratus cannot.