We also observed a population of naïve CD4 T cells (denoted ‘Xcl1+ naïve’) expressing Lef1 and Sell genes along with Xcl1, which is a ligand of XCR1 (or G protein-coupled receptor 5, GPR5), with increased expression on NK and CD8+ T cells during virus infection. This evidence concerns the gene XCR1 and viral infectious disease.