Two small clinical trials revive hope for an old idea: Cells injected into the brain might replace the nerve cells that die in Parkinson’s disease. The studies, published in Nature, represent early steps for stem cell therapies that aim to replace these dead cells in the brain —and stop Parkinson’s and the movement problems, tremors and rigidity that it brings.

In both trials, scientists injected cells derived from stem cells that would go on to become specialized neurons that pump out the chemical messenger dopamine. These are the crucial cells in the brain that die in Parkinson’s disease, a relentless neurological disease that is estimated to affect more than 8 million people worldwide.