Psychiatric disorders such as autism, schizophrenia, and depression share similarities at the molecular level, according to a new analysis of 700 donated brains.

It is well-known that genetics can make people more susceptible to psychiatric disorders, but the link is more complicated. DNA includes instructions for making proteins, but these instructions can be carried out in different ways by another genetic building block called RNA. (This is called “gene expression.”) In a study published today in the journal Science, researchers analyzed the brains of 700 people who had autism, schizophrenia, alcoholism, bipolar disorder, and major depressive disorder. The RNA molecules in these brains had all been sequenced into something called a transcriptome. By comparing these transcriptomes to each other (and against the transcriptomes of people without the disorders) the scientists found important physical similarities.

There was the most overlap between those with schizophrenia and bipolar disorder. Plus, these two disorders and autism were associated with extra sensitivity in brain cells called astrocytes. Depression, on the other hand, didn’t show this, but it had a distinctive pattern of hormone signaling that none of the others did. The scientists found the least amount of overlap between autism and alcoholism, and schizophrenia and alcoholism.

To make sure that the results weren’t due to medication, the scientists compared these brains with the brains of non-human primates who had been given antipsychotics. The comparison found that antipsychotics aren’t likely to create these differences.

We still don’t know what’s causing these differences in gene expression, but the study is an interesting step forward in understanding the biological component of psychiatric illness. “The major challenge is to understand how diverse genetic and environmental causes lead to these changes, but knowing that these patterns exist is a key step in that critical process of understanding disease mechanisms,” study co-author Daniel Geshwind, a psychiatrist at UCLA, told The Verge via email. If we more fully understand these biological changes, we may be able to more accurately diagnose these disorders and create better medications to treat them.