© 2016 by The American Association of Immunologists, Inc. The human homologue of Drosophila Toll (I1T0II), also called Toll-like receptor 4 (TLR4), is a recently cloned receptor of the IL-1/Toll receptor family. Interestingly, the TLR4 gene has been localized to the same region to which the Lps locus (endotoxin unresponsive gene locus) is mapped. To examine the role of TLR4 in LPS responsiveness, we have generated mice lacking TLR4. Macrophages ami B cells from TLR4-deflcient mice did not respond to LPS. All these manifestations were quite similar to those of LPS-hyporesponsive C3H/HeJ mice. Furthermore, C31 I/He J mice have, in the cytoplasmic portion of TLR4, a single point mutation of the amino acid that is highly conserved among the IL-l/Toll receptor family. Over- expression of wild-Type TLR4 but not the mutant TI.R4 from C3H/HeJ mice activated NF-kB. Taken together, the present study demonstrates that TLR4 is the gene product that regulates LPS response. The Journal of Immunology, 1999. 162: 3749-3752.