PDCD1 and neoplasm: To validate this tumor-retention mechanism, we sorted CXCR6+PD-1+ and IL7R+IL18R1+ precursor populations from the tumors of CD45.1+ congenically marked BALB/c mice and intratumorally transferred equal numbers (15,000–25,000) into a 4T1 tumor growing in the mammary tissue of Rag2KO BALB/c mice (Figure 6E).