TOX3, also known as TNRC9 (trinucleotide repeat containing 9), is a neuronal transcription factor containing a nuclear localization signal and a high mobility group (HMG)-box domain followed by a C-terminal poly-glutamine stretch. TOX3 is specifically expressed in the estrogen receptor alpha positive (ER+) subset of murine mammary luminal epithelial cells, including a recently identified progenitor cell subset (PMID: 19196971). TOX3 expression is negatively correlated with the immunotherapy targets PD-1, PD-L1 and HAVCR2 in lung adenocarcinoma, indicating that it serves a role in immunity activation (PMID: 27080130).