The study investigates the role of human Toll-like receptor 9 (hTLR9) in recognizing and responding to bacterial deoxycytidylate-phosphate-deoxyguanosine (CpG)-DNA. It finds that hTLR9 expression in human immune cells correlates with responsiveness to CpG-DNA, and that transfection of hTLR9 into nonresponsive cells can confer responsiveness. The optimal CpG motif for hTLR9 is GTCGTT, while the optimal murine sequence is GACGTT. These findings suggest that hTLR9 directly engages immunostimulatory CpG-DNA, providing a species-specific mechanism for CpG-DNA recognition and signaling in human cells.The study investigates the role of human Toll-like receptor 9 (hTLR9) in recognizing and responding to bacterial deoxycytidylate-phosphate-deoxyguanosine (CpG)-DNA. It finds that hTLR9 expression in human immune cells correlates with responsiveness to CpG-DNA, and that transfection of hTLR9 into nonresponsive cells can confer responsiveness. The optimal CpG motif for hTLR9 is GTCGTT, while the optimal murine sequence is GACGTT. These findings suggest that hTLR9 directly engages immunostimulatory CpG-DNA, providing a species-specific mechanism for CpG-DNA recognition and signaling in human cells.