ITPRIPL1 (Inositol 1,4,5-Trisphosphate Receptor-Interacting Protein-Like 1) is a single-pass type I membrane protein that functions as an inhibitory ligand of CD3ε, suppressing T cell activation 1. The protein binds CD3ε on T cells, inducing stable CD3E-NCK1 binding that prevents CD3E-ZAP70 interaction, thereby blocking ERK-NFκB signaling and reducing calcium influx 1. This mechanism impedes T cell activation during the critical "signal one" phase of TCR signaling 1. Clinically, ITPRIPL1 promotes tumor immune evasion across multiple solid tumors by suppressing effector T cell responses while promoting interactions with regulatory T cells and exhausted T cells 23. Elevated ITPRIPL1 expression correlates with higher tumor grade, poorer outcomes, and reduced infiltration of effector memory CD4+ T cells and Th17 cells, while increasing M2-polarized macrophages and immune checkpoint molecules 23. Notably, elevated expression serves as a prognostic biomarker across glioma, gastric adenocarcinoma, and lung adenocarcinoma 24, though it correlates with better prognosis in breast cancer 5. Neutralizing anti-ITPRIPL1 antibodies restrain tumor growth and enhance T cell infiltration, positioning ITPRIPL1 as a promising immunotherapeutic target 1.