As expected, tumor-infiltrating Il9–/– or Il9r–/– Tc9 cells had higher lipid peroxidation (Figure 6A) and cellular iron levels (Figure 6B), and lower mitochondrial activity (Figure 6C) than WT Tc9 cells, indicating that IL-9– or IL-9 signaling–deficient Tc9 cells undergo enhanced ferroptosis in the TME. This evidence concerns the gene IL9R and neoplasm.