Day One Biopharmaceuticals reported Sunday that an experimental molecule shrank tumors by at least 50% in a majority of children with the most common form of childhood brain cancer, offering an early but rare glimmer of progress in a disease that has seen little over the last three decades. In the Phase 2 trial, 22 patients ages 3 to 18 with low-grade glioma who had progressed on at least three previous treatments were given a targeted drug called tovorafenib. Of those, 14 — or 64% — saw their tumors wither to less than half their size. Six others had stable disease.