Williams Syndrome is another example. I have a friend who seems always happy, calm and overall a very nice guy. I’ve never heard him curse or witnessed him being violent, and he says that he never was like that. My other friends and I realize that there is something different about him, but being “different” is not something new. We also knew other people like him, so it wasn´t a big deal.
Then one day I saw this clip and I realized then that he has the same symptoms as the Williams Syndrome suggests: “distinctive, "elfin" facial appearance, along with a low nasal bridge, an unusually cheerful demeanor and ease with strangers; developmental delay coupled with strong language skills; and cardiovascular problems”.
However, “Williams syndrome is caused by the spontaneous deletion of genetic material from the region q11.23 of chromosome 7. The deleted region includes more than 25 genes, and researchers believe that the loss of several of these genes probably contributes to the characteristic features of this disorder”
So, again, this is an example of another bio-chemical mark of a specific disease. We would have been able to tell he has Williams Syndrome just by looking at his behaviour, but you can test that in a scientific manner to confirm it, unlike a “mental disease”.