There is even some evidence that ORs can distinguish isotopologues: molecules with the same chemical formula and connectivity, differing in only their isotopic composition. Fruit flies and honeybees can be trained to distinguish between hydrogenated and deuterated odorants like acetophenone. However, these results do not extend directly to vertebrates because of the difference the two groups' olfactory systems; insect olfactory receptors are (likely) not even G protein-coupled receptors . Trials in humans, both in vitro and in living subjects , are inconclusive. The extreme sensitivity of the olfactory system to impurities and the subjective nature of smell tests complicates these experiments.