Sad delay in Parkinson’s tarnishes MICHAEL J. FOX’s happy five-oh.

As Michael approaches his 50th birthday, he is coming to the sad realization that he will likely never see a cure for Parkinson’s disease in his lifetime.

The actor, who will reach the milestone birthday on June 9, puts on a brave public face, but insiders say he was devastated by a recent setback in Parkinson’s research.

Many scientists thought that a promising new type of stem cell – called pluripotent stem cells – would eventually lead to a cure for Parkinson’s. But in an unexpected blow, new research has found that the cells won’t be effective because they will be rejected by patients’ immune systems.

“It’s a crushing blow to Michael and some of the research that his foundation is funding,” a close source told The ENQUIRER.

The former “Spin City” actor, who was diagnosed with the deadly disease in 1991, suffers from tremors, slurred speech, limping and the dragging of his left foot due to the degenerative neurological disorder.

While there are drugs that can treat these symptoms, many can have severe side effects and none can stop the disease from progressing.

In 1998, Michael had brain surgery to alleviate the tremors, but Dr. A. Jon Stoessl, director of the leading Pacific Parkinson’s Research Center at the University of British Columbia, said bluntly: “I am very reluctant to say that Michael will see a cure to this disease in his lifetime.”

In 2007, The ENQUIRER reported exclusively that through his Michael J. Fox Foundation for Parkinson’s Research, the actor green-lighted funds of more than $1 million to two Israeli-based companies working on cutting-edge treatments.

Two years later, we reported that scientists at the Whitehead Institute for Biomedical Research in Cambridge, Mass., announced a breakthrough in the fight against Parkinson’s disease – they discovered how to replace impaired brain cells in Parkinson’s patients with disease-free ones.

But a cure continues to elude researchers.

“Michael is seeing his life ebb away,” continued the source. “Turning 50 is a wake-up call. He wonders if he’ll be able to dance at his kids’ weddings.

“Michael had such high hopes for this potentially miraculous stem cell therapy, but now it’s back to the drawing board.”