A Swiss toddler who received a personalized RNA drug for a genetic form of epilepsy died after developing hydrocephalus, a buildup of fluid in the brain, The New York Times reports. The girl initially had fewer seizures while taking the experimental drug, but died a year later at age 3, researchers reported at a 23 October session at the annual meeting of the American Neurological Association in Chicago. Another girl receiving spinal fluid injections of the same drug also developed hydrocephalus but is still alive. These types of custom-made drugs that modify gene expression, known as antisense oligonucleotide (ASO) drugs, have been seen as a promising way to treat some people with extremely rare genetic diseases. But the NYT reports that hydrocephalus has also been seen in patients receiving ASO treatments for the brain disorder disease, as well as the muscle wasting disease spinal muscular atrophy, and these cases could point to a broader problem with the approach.