They had promis­ing gene ther­a­pies for their daugh­ter's ul­tra-rare dis­ease. But how to tell if one worked?

About two years ago, Ma­jid Ja­far, CEO of the 50-year-old Emi­rati oil firm Cres­cent Pe­tro­le­um, found him­self with a prob­lem of his own mak­ing, one that he was hap­py to have.

In 2014, Ja­far and his wife, Lynn Bargh­out Ja­far, had a daugh­ter, Alia. She came home more than 8 pounds and healthy. But on day 19, Alia suf­fered 10 seizures. The Ja­fars rushed her to doc­tors, who sev­er­al months lat­er fi­nal­ly di­ag­nosed her with CD­KL5 de­fi­cien­cy dis­or­der, or CDD, a dis­ease that just a decade pri­or re­searchers didn’t know ex­ist­ed.

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