Okay, so this is pretty obscure. But it's a good example of how medicine advances.
Wolfram syndrome is a very rare condition, with a prevalence of about one in a million. It is very disabling: diabetes mellitus in childhood, optic atrophy leading to blindness, diabetes inspidus, ataxia and neuromuscular bladder problems. Currently no treatment is available for the underlying disease.
But, and here the science comes in, the causative gene has been identified, and a 'knockout' mouse developed (i.e. a mouse missing the relevant gene). The molecular fault is known: a transmembrane protein that is induced in the endoplasmic reticulum in response to stress. Here comes the clever bit: because of research on bipolar disorder, sodium valproate is known to affect this pathway. So maybe it will work in Wolfram syndrome??
That's known a 'repurposing': old drugs for new diseases. The key to success is a thorough understanding of the molecular mechanisms of disease.