While an interview with Colin Farrell is generally a jocular affair, the actor lately has been injecting a more serious tone to his moments in the spotlight.

While on a promo run for his new vampire flick, "Fright Night," the 35-year-old revealed that his oldest son, James, suffers from a rare condition called Angelman syndrome.

“James, just a quick shout out if I can, James has a condition called Angelman syndrome and it only affects one in 30,000 children, they say,” Farrell told David Letterman. “So there’s not that much funding because it doesn’t benefit pharmaceutical companies, of course.”

The neuro-genetic disorder is caused by the loss of the normal maternal contribution to a region of chromosome 15. Children diagnosed are often unusually happy, with frequent laughter or smiling. They may also need less sleep than normal and experience intellectual and developmental delay, seizures, and jerky movements.

Farrell has been encouraging viewers to visit CureAngelman.org, a nonprofit foundation working on a cure to the syndrome through gene therapy. "We know what is involved," the organization says on their site. "The research suggests that this can be done. And we have real ways to accomplish this goal from a scientific standpoint. The only thing standing between individuals with Angelman syndrome and a cure is money."

With the right funds, the organization believes that treatment could happen within the next 10 years.

Check out this video of Farrell discussing his son on "Late Night" below.

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