CD4 and breast neoplasm: To build this case, we showed that CD4+ naive T cells are abundant in the stroma of extensively perfused samples — close to, but not within blood vessels — in most breast tumors compared to adjacent normal tissue; that their abundance correlates with TI Treg numbers; that the clonotypic TCR sequences of TI naive CD4+ T cells, rather than PB Tregs or TI memory T cells, are most similar to that of TI Tregs; and that the abundance of naive CD4+, but not CD8+, T cells in tumors is an independent negative prognostic factor for breast cancer.