TarGeT-D is a molecularly stratified therapeutic arm of the CONNECT TarGeT umbrella protocol. This Phase II study evaluates the efficacy and safety of olutasidenib, a selective oral inhibitor of mutated IDH1, administered in combination with temozolomide for paediatric and young adult patients with newly diagnosed IDH1-mutant HGG or DIPG. The primary objective is to determine if this targeted combination improves clinical outcomes following radiotherapy. Correlative science includes analysing molecular markers of response, validating findings via neuroimaging, and evaluating the incidence of pseudoprogression to refine precision oncology strategies for this genetic subset.