An Ox40-cre allele was used for lineage marking of CD4 T cells. Naive T cells that had previously expressed OX40 demonstrated a partially activated phenotype that was distinct from that of the majority of naive T cells. These results are consistent with a minor subpopulation of naive T cells being dependent on strong signaling responses to thymic self ligands. Keywords: T cell population comparison. Characterization of a novel subpopulation of naive T cells. Four types of T cells were sorted from mice (naive, CD44lo, CD44hi and Treg).