He’s 19 months old and has one of the world’s rarest diseases. His parents are determined to find a cure.

SOMERVILLE — At 19 months old, Lucas Guo has never crawled, walked, or stood by himself. He babbles but can’t say words. He struggles to chew and swallow. He gets most of his nutrition through a feeding tube attached to his belly.

Lucas has one of the world’s rarest diseases, ZTTK syndrome, a complex neurological disorder caused by a genetic mutation. Scientists have documented only about 60 cases worldwide and estimate that, at most, it afflicts several hundred people — far too few to encourage drug makers to find a treatment.

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That economic reality has driven Lucas’s parents, Ada Lio and Nathan Guo, to take things into their own hands. Working through a small foundation, they are trying to raise $10 million to develop a treatment or cure for ZTTK in five years.

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