They cultured PDOs derived from the tumor tissues of 30 patients with HGSOC in a medium supplemented with anti-PD-1 antibodies and found that PDOs with lower levels of stromal tumor-infiltrating mast cells (TIMs) exhibited superior effector functions of CD8+ T cells in response to PD-1 blockade, indicating that PD-1 blockade could restore dysfunctional CD8+ T cells in patients with low stromal TIMs. The gene discussed is CD8A; the disease is neoplasm.