Beckham O'Neill is only 7, but he takes his big brother role very seriously.

Looking into the camera for the family's second video plea for donations, Beckham explains that his 4-year-old sister, Eliza, has Sanfilippo syndrome, which "clogs up her brain, and that makes her not learn very well."

"We really need you to help us get my sister's medicine for her immediately," he says, before the screen fades to black and the following words appear:

"He does not know the disease is terminal."

Dad Glenn O'Neill said watching the video makes him and his wife cry every time.

Read our story about Eliza's diagnosis and her parents' race for a cure.

Eliza has Sanfilippo syndrome type A, a rare genetic disorder that causes a deadly buildup of heparin sulfate in her cells. Soon, Eliza will lose the ability to speak, her parents say. After that, she'll lose the ability to walk and then she'll develop seizures.

Most children with Sanfilippo type A don't live far into their teens.

All varieties of the disease affect one in 70,000 live births, according to the National Institutes of Health, but there is no cure.

Doctors at Nationwide Children's Hospital in Columbus, Ohio, have been working on a cure for 16 years and have found a gene therapy that works in mice. They hope to try it on humans early next year, but they need about $2 million to make a clinical trial happen.

That's where the O'Neills and other families battling Sanfilippo come in.

Read about how the O'Neill's first video brought in more than $500,000 in 15 days.

Beckham O'Neill, 7, is asking for help to fund research for his little sister's deadly disease. (Courtesy O'Neill Family and Von Wong)