The SJiMB21 study is a clinical trial for infants and young children (under age 5) newly diagnosed with medulloblastoma, a fast-growing brain tumour. Because radiation can be very harmful to a young child’s developing brain, this study uses a “molecularly-guided” approach to tailor treatment based on the tumour’s specific genetic group. Using a cutting edge method called DNA methylation, doctors identify the tumour’s unique biological signature. For some groups, patients may be treated with chemotherapy alone to avoid radiation entirely. For others, chemotherapy is used to safely delay radiation until the child is 36 months old, allowing for more brain development. The goal is to improve survival while protecting the child’s learning and thinking processes.