Background: Recently identified T memory stem (Tscm) cells have stem-cell-like properties including long-lasting,self-renewal capacity,and multipotency to differentiate into other memory T cell types.Selective depletion of CCR5+CD4+ Tscm cells and the high proliferation rate of these cells are thought to be responsible for the pathogenesis of simian immunodeficiency virus (SIV) infection in rhesus macaques.