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A 10-episode miniseries titled “American Sports Story: Aaron Hernandez” debuts Sept. 17 on FX. It explores the rise and fall of NFL player Aaron Hernandez — the New England Patriots tight end who was convicted of murder in 2015 and committed suicide in his prison cell two years later.

Hernandez was diagnosed after his death with chronic traumatic encephalopathy, also known as CTE. The disease can cause impulsive and aggressive behavior, emotional instability and suicidal thoughts, according to the Mayo Clinic.

A researcher who studied Hernandez’s brain said at the time that he had the most severe form of the brain disorder she had ever seen in someone his age, raising questions about what caused his violence. Hernandez was 27 when he died. Researchers have not made a direct connection between Hernandez’s illness and his behavior.

Here’s what you need to know about CTE and Hernandez’s brain.

What is CTE?

CTE is an incurable degenerative brain disorder that causes the death of nerve cells in the brain due to repeated head injuries, according to the Mayo Clinic. It doesn’t appear right away and gets worse over time, according to the Cleveland Clinic. The condition can lead to behavioral changes, cognitive impairment and progression to other brain disorders, such as dementia.

CTE usually starts in the front of the brain, where most head trauma occurs, and then spreads to other areas over time, Dr. Jesse Mez, co-director of clinical research at Boston University’s Chronic Traumatic Encephalopathy Center, tells TODAY.com.

The effects of CTE can take years to show because the disease develops slowly, Mez says. However, more repeated head injuries can make it spread faster, as can aging and being more genetically susceptible to CTE.

CTE has not been studied as thoroughly as other degenerative brain diseases, Dr. Daniel Daneshvar, chief of the division of brain injury rehabilitation at Massachusetts General Hospital, tells TODAY.com. That’s partly because CTE can only be diagnosed after death, since the brain must be examined under a microscope.

Therefore, according to Julie Stamm, Ph.D., a neuroscientist at the University of Wisconsin-Madison who studies sports-related brain injuries, including CTE, it’s impossible to know how common the condition actually is, she tells TODAY.com.

People who develop CTE often have a history of playing professional sports, according to the Cleveland Clinic. Boxers and football players have the highest number of confirmed cases of CTE, but it has also been found in soccer players, ice hockey players, wrestlers, rugby players and more, according to research.

In addition to athletes, military veterans exposed to explosives and individuals who have been repeatedly physically abused are also at greater risk of developing CTE, Daneshvar said.

While many questions remain about CTE, there is one finding that links all the documented cases: “a previous history of brain injury or repeated head blows,” Dr. Willie Stewart, a neuropathologist at Queen Elizabeth University Hospital in Glasgow, Scotland, tells TODAY.com.

How does CTE affect a person’s behavior?

CTE can affect a person’s behavior in several ways.

“Memory loss and cognitive changes are the most predictive symptoms of CTE, but other common symptoms include impulsivity, aggression, verbal and physical abuse, short temper, loss of control and depression,” Dr. Ann McKee, a neuropathologist and director of the Chronic Traumatic Encephalopathy Center at Boston University, tells TODAY.com.

“CTE has the greatest effect on the frontal cortex of the brain, which plays an important role in impulse control and other behaviors,” Mez says.

However, it is too early in the research to definitively prove that CTE is the cause of the problematic behavior of some people with CTE, Mez and Nowinski emphasize.

What Did Aaron Hernandez’s Brain Scans Reveal About His CTE?

McKee, who studied Hernandez’s brain, says she has examined more than 700 cases of CTE and found that Hernandez’s brain was “the worst CTE I’ve ever seen in someone who died at that age (27).”

Hernandez’s brain was donated by his family to Boston University’s Chronic Traumatic Encephalopathy Center, where it was extensively studied. McKee and her colleagues found that he had stage 3 CTE, with stage 4 being the most severe and usually found in people who were much older.

Research also shows that the longer someone plays tackle football, the greater their risk of developing CTE. Hernandez played football as a teenager, the New York Times reported.

But it’s still impossible to say whether the severity of his CTE caused his violent behavior. “We can’t draw the line between pathology in someone’s brain and specific actions that someone takes,” Daneshvar says.

McKee added: “It’s impossible to know exactly how the CTE affected him, as many factors likely contributed to his behavior and actions. … CTE likely contributed to his depression and his erratic, impulsive, violent behavior.”

How did Aaron Hernandez die?

Aaron Hernandez was found dead in his cell in April 2017, apparently by suicide. He was serving a life sentence in Massachusetts after being convicted of the murder of Odin Lloyd.

Research from the Mayo Clinic shows that people with CTE have a greater risk of suicide.