In the late 1980s and over the 1990s, researchers pioneered the transplantation of new nerve cells into the brains of patients with Parkinson's disease. The outcomes proved for the first time that transplanted nerve cells can survive and function in the diseased human brain. Some patients showed marked improvement after the transplantation while others showed moderate or no relief of symptoms. A small number of patients suffered unwanted side-effects in the form of involuntary movements.