The award-winning actor attended an event at the Chicago Hyatt Regency which raised money to battle the illness.
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"It's a heart opener this evening such a heart opener...I look forward to this day every year," Farrell said.
Farrell, whose latest film is "Fantastic beasts and where to find them," has been coming for years to the fundraiser.
"When you become a parent...you want your child to be special and then you find your child is really special and it can really kick you...it can really shock the hell out of you," Farrell said.
Angelman Syndrome is a rare neuro-genetic disorder that affects one in 15,000. It's sometimes misdiagnosed as cerebral palsy or autism.
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"There are about 7,000 rare disorders out there. it's hard to shine a light on each and everyone. But Angelman's Syndrome is getting a lot of attention now just because of what causes the disorder and how close we are to treating it and curing it," said Paula Evans, founder of the Foundation for Angleman Syndrome Therapeutics.
Families from all over the world came to Chicago for the event. Each had an opportunity to meet a number of celebrities connected somehow to the syndrome, including former Disney Channel star Josh Peck whose manager has a child with Angelman Syndrome.
"This is my first year. Getting to come to Chicago and meet the families and hear some of the science and hear how the disease is being treated ... it's really exciting and gives a lot of hope," Peck said.