When Richard Finnell first met her, Rachel was a nine-year-old girl with severe developmental delays. Her condition seemed to be caused by a deficiency in a critical B vitamin called folate. Yet she had plenty of folate circulating in her blood. Somehow it was vanishing before it got to her brain and spine. Eventually Finnell made a surprising discovery. He's now using new genetic tools like CRISPR to better understand her condition and test possible therapies.