THEMIS (thymocyte selection associated) is a T-lineage-restricted protein that plays a critical role in T cell development during thymocyte positive selection 1. The primary function of THEMIS is to enhance T cell antigen receptor (TCR) signaling sensitivity, enabling thymocytes to reach the activation threshold required for positive selection 2. Mechanistically, THEMIS suppresses the activity of the tyrosine phosphatase SHP1, rendering CD4+CD8+ thymocytes more responsive to low-affinity self-ligand interactions 2. Additionally, THEMIS modulates immune checkpoint signaling by recruiting to the cytoplasmic domain of BTLA and promoting oxidation of SHP-1's catalytic cysteine, thereby increasing the signaling threshold for BTLA-mediated T cell inhibition 3. This dual mechanism allows THEMIS to support both thymocyte positive selection and peripheral CD8+ T cell maintenance. In disease contexts, reduced THEMIS expression (THEMIShi CD8+ T subtype absence) correlates with diminished immunotherapeutic responses in hepatocellular carcinoma patients 4. Similarly, in non-small cell lung cancer, decreased transition of T_THEMIS cells to regulatory T cells associates with improved therapeutic responses to neoadjuvant combination therapy 5. These findings suggest THEMIS regulates critical checkpoints in T cell development and anti-tumor immunity.