Gallery: NFL players with confirmed brain injury
MALIBU, CA - JULY 24, 2009: Hall of Fame linebacker and San Diego's own Junior Seau suffered from CTE. (Photo by Frazer Harrison/Getty Images)Photo by: Frazer Harrison
Gallery: NFL players with confirmed brain injury
Chicago Bears defensive back Dave Duerson of the Chicago Bears works against the Detroit Lions during a game at Soldier Field in Chicago, Ill. Duerson committed suicide in 2011 and later was confirmed to have CTE. Photo by: Jonathan Daniel
Gallery: NFL players with confirmed brain injury
Tailback Frank Gifford of the University of Southern California. The NFL star and revered sportscaster had CTE. (Photo by University of Southern California/Getty Images)Photo by: University of Southern Californi
Gallery: NFL players with confirmed brain injury
Coconut Grove - Feb. 1, 2007: Pro Football Hall of Famer John Mackey and his wife Sylvia attend party during Super Bowl XLI weekend (Photo by Brian Bahr/Getty Images for PLAYERS INC) Mackey suffered from symptoms of dementia. His brain showed confirmed signs of CTE. Photo by: Brian Bahr
Gallery: NFL players with confirmed brain injury
Sep. 13, 1992: Denver defensive lineman Shane Dronett rushes around the outside of San Diego Chargers offensive line at Mile High Stadium. Dronett's mother spoke to media after he died about the havok CTE wreaked on her son. Photo by: Tim DeFrisco
Gallery: NFL players with confirmed brain injury
Cincinatti 2005: Chris Henry of the Cincinnati Bengals poses for his 2005 NFL headshot at photo day. (Photo by Getty Images) Henry died when he fell from a truck. He was believed to be the first still-active NFL player to have CTE.Photo by: NFL Photos/Getty Images
Gallery: NFL players with confirmed brain injury
Nov. 25, 2012. Kansas City, MO. Kansas City Chiefs inside linebacker Jovan Belcher #59 watches from the sideliens during his final game against the Denver Broncos at Arrowhead Stadium (Photo by Jamie Squire/Getty Images). Belcher shot and killed his girlfriend before committing suicide. The 25-year-old's brain had signature signs of CTE. Photo by: Jamie Squire
Gallery: NFL players with confirmed brain injury
Nov. 21, 1999: Philadelphia Eagles Kevin Turner #34 walks on the field during the game against the Indianapolis Colts at the Veterans Stadium. Mandatory Credit: Doug Pensinger /Allsport. Turner had the most advanced stage of CTE when he died at age 46. Photo by: Doug Pensinger
Gallery: NFL players with confirmed brain injury
Sep. 6, 1992: Philadelphia Eagles defensive back Andre Waters sits on the bench during a game against the New Orleans Saints at Louisana Superdome Mandatory Credit: Gene Sweeney /Allsport. CTE may have led to Waters' depression and suicide.  Photo by: Gene Sweeney
Gallery: NFL players with confirmed brain injury
Sep. 18, 1988: New York Jets running back Johnny Hector moves the ball as Houston Oilers attempt to tackle him during a game at Giants Stadium. He was the first deceased athlete examined by CTE researchers. Photo by: Rick Stewart
Gallery: NFL players with confirmed brain injury
1 Aug 1998: Center Dermontti Dawson #63 of the Pittsburgh Steelers standing with Tackle Justin Strzelczyk and Guard Brenden Stai #68 on the sideline during the Pro Football Hall of Fame Game against the Tampa Bay Buccaneers at the Fawcett Stadium in CantPhoto by: Stephen Dunn