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About one in 21,000 boys are born with ALD, which is caused by a flaw in a gene on the X chromosome that prevents cells from making a protein that the cells need to process certain fats—females have a backup copy of the gene on their second X chromosome. Without that protein, the fats build up and gradually destroy myelin sheaths that protect nerves in the brain. In the cerebral form of ALD, which begins in childhood, patients quickly lose vision and mobility, usually dying by age 12.

The scientists removed blood cells from 17 boys with cerebral ALD ranging from age 4 to 13 and treated the cells with a modified HIV virus carrying the gene for the ALD protein. They then reinfused the cells back into patients.
[...] In brain scans taken up to 2 years later, most had no further signs of inflammation or loss of myelin. And 16 of the 17 had no signs of neurological decline such as vision loss or trouble walking.

Although European authorities have already given the nod to a gene therapy for a rare disorder and are expected to soon approve a second, FDA has yet to approve a single gene therapy.