Sought Out by Science, and Then Forgotten

José Echeverría spends restless days in a metal chair reinforced with boards and padded with a piece of foam that his mother, Nohora Vásquez, adjusts constantly for his comfort. The chair is coming loose and will soon fall apart. Huntington’s disease, which causes José to move his head and limbs uncontrollably, has already left one bed frame destroyed. At 42, he is still strong.

José’s sister Nohora Esther Echeverría, 37, lives with her mother and brother. Just two years into her illness, her symptoms are milder than his, but she is afraid to walk around her town’s steep streets, knowing she could fall. A sign on the front door advertises rum for sale that does not exist. The family’s scarce resources now go to food — José and Nohora Esther must eat frequently or they will rapidly lose weight — and medical supplies, like a costly cream for Jose’s skin.

Huntington’s is a hereditary neurodegenerative disease caused by excess repetitions of three building blocks of DNA — cytosine, adenine, and guanine — on a gene called huntingtin. The mutation results...