An extreme example comes from a study that described a mutation in the enzyme that manufactures the brain chemical serotonin (the neurotransmitter on which antidepressants like Prozac ACTS).

She found that worms with a mutation in a gene called odr-10 could not smell diacetyl, a chemical that gives butter its odor and is also produced by a bacterium that is a favorite worm food.

One of the original bugs had undergone a mutation that caused it to excrete acetate, an organic chemical.