Attention on brain injuries in sports has been focused in recent years on boxers, such as Sugar Ray Robinson, Jerry Quarry and especially Muhammad Ali, whose Parkinson's disease may have been exacerbated by repeated blows to the head.

But plenty of other athletes have been forced to retire after suffering concussions, including hockey's Pat LaFontaine and Brett Lindros and football's Merril Hoge and Al Toon.

The problem, doctors say, is that no one is quite sure how many are too many -- or when the next hit could be the blow that takes away an athlete's motor skills and memory.

"I don't know how many athletes have sat in front of me and said, `Doc, what is down the road for me if I've had so many concussions?"' said Dr. Michael Collins of the Division of Neuropsychology at Henry Ford Health System in Detroit.

"We just don't know," he said.

Young, 38, was knocked out during a game against Arizona on Sept. 27. It was his fourth concussion in three years. He will sit out the next two games, and his diagnosis will be reevaluated at that time.

Young's neurologist, Dr. Gary Steinberg of the Stanford University School of Medicine, categorized the concussion as mild and said there was no indication of permanent brain damage.

"I don't think we're talking about a Muhammad Ali-type of situation here," Steinberg said. "It's nothing of the order of magnitude compared to Muhammad Ali."

Doctors say that Ali and other boxers helped bring attention to the potential problems of brain injury in sports. Robinson and Quarry both showed signs of dementia pugilistica, or being "punch drunk." Quarry, a hard-punching heavyweight who fought Ali and Floyd Patterson, could not dress or feed himself when he died earlier this year.

Dr. Collins led a study published last month in the Journal of the American Medical Association showing that college football players who sustain multiple concussions are at risk for a decline in long-term neuropsychological function.

Additionally, players with a learning disability have an even higher risk for permanent injury from multiple concussions, the study said.

But research on concussions is scarce and athletes have, in general, been taught to ignore some of their injuries -- especially those that aren't visibly apparent.

"We are trained as a society that if you have a mild ankle injury, or a mild groin pull, you're trained to play through the pain," Collins said. "But I think that when you have an injury that involves the brain, you have to look at it differently."

Professional athletes and their leagues are starting to take notice. The NFL and NHL have programs to address concussions.

Earlier this season, Mike Modano of the NHL's Dallas Stars was checked head-first into the boards by Anaheim defenseman Ruslan Salei. A frightening episode -- but it wasn't enough to keep him from playing.

Modano, who was knocked cold, suffered a mild concussion and strained neck ligaments, returned to the ice despite the threat of debilitating injury. But it has given him pause. "I still have the rest of my life to live," he said.

Young has told 49ers coach Steve Mariucci and general manager Bill Walsh that he wants to play again.

But there is evidence that once an athlete has a concussion, he's more likely to get another one, said Dr. Martin Holland, director of the University of California, San Francisco, Brain and Spinal Injury Program and a neurosurgeon at San Francisco General Hospital.

There are two theories to support this: first, athletes who are risk-takers tend to have a greater chance of getting concussions, and second, a concussion can cause enough of an impairment that reaction is dulled.

"Steve Young is at higher risk for concussions," Holland said. "There's nothing that tells me he won't have a fifth or a sixth or a seventh if he continues to play."

That's one of the reasons Young should take a hard look at retirement, Holland said.

"My personal opinion is that he should probably quit. Not necessarily because the next one could do him in, but that he's already had four concussions and he's at risk to have more," he said. "He has too much lose."