"To say that cord blood stem cells are ready for primetime, ready for interventions in neurodiseases or heart diseases, or anything outside the blood, is really the height of speculation," Daley said.
In the United States, cord blood stem cells have only been used to treat blood-related diseases, such as leukemia.
Yet, on Beike's Web site, treatable conditions run the gamut: autism, Amyotrophic Lateral Sclerosis (known as "Lou Gehrig's disease"), Parkinson's disease and Optic Nerve Hypoplasia.
Hu disagrees with U.S. experts who say there is no evidence that umbilical cord stem cells can help people with the sort of neuromuscular diseases that he is treating.
"That's wrong," Hu said. "There's no problem [with] that. And there are many, many publications."
Daley, however, is skeptical.
"I am familiar with a couple of publications that have reported that umbilical stem cells, UCB stem cells, have been used in animal models of stroke," he said. "These are incredibly preliminary types of experiments. They would, by no means, establish enough of a proof of principal to justify going into human patients with these kinds of therapies. I would be very suspicious."
A Beike spokesperson told "Nightline" in response that, "If you get too caught up on the controlled study subject, and the difficult process of getting scientific data from China published in Western journals, you are going to miss the story, though. For most of the patients out there, getting improvement in their condition takes precedence over reading a double-blind study."
Hu described the high costs of doing a clinical trial, and pointed out that many patients are eager for treatment, regardless of whether it has been clinically proven.
This reasoning worries Daley.
"I'd be very concerned if a company is doing clinical interventions under the promise of medical cures, in a way where they are not gathering data in a reliable, responsible, controlled manner that's going to allow for the rest of the medical community to learn from it," he said.
But according to Hu, most of the patients do get improvements from the treatment.
"We are not talking about cure, but improvements," he said.
The mention of improvements, however, was merely "a matter of semantics" to Daley, especially if Beike charges patients tens of thousands of dollars.
"I think that this is highly unethical. Certainly does not meet the medical standards that we would practice here in the United States," he said.
Despite the running debate, Flynn stands by his decision. In October, he returned to China for a second treatment. And this time, his older brother Bob came along.
Flynn and his family are convinced by what they see, and even sent "Nightline" video diaries after their October trip.
On screen, Flynn illustrated his progress.
"It's a bunch of little things that are starting to reconnect together, like I can do this now." Flynn held his hand up, and separated his middle finger from his ring finger. "I sure wasn't able to do that earlier, put my fingers out like [a Star Trek] Vulcan.
"Little things like that. I can jump now, which I couldn't jump earlier. I can jump now, little frog hops, which requires all the contraction and releasing and coordination of all of your leg muscles all at once. So, just little things like that. Squatting, I can squat a little better now without falling over."