The brain tumor afflicting Sen. Edward Kennedy — a glioblastoma — is the most aggressive form of brain cancer. But scientists have discovered the tumor’s vulnerability. They have identified a network of genes that create the perfect environment to allow the tumor to mushroom to the size of an apple in a just a few months. Researchers also identified a new gene whose level in the tumor predicts how long a glioblastoma patient will survive. The discoveries offer new targets for therapies.