Defects in the gene encoding Toll-like receptor 4 (Tlr4) result in impaired responses to lipopolysaccharide (LPS), rendering mice sensitive to infections by Gram-negative bacteria. C3H/HeJ mice have a codominant allele with a mutation in Tlr4, which results in an intermediate response to LPS in F₁ mice from crosses of responder and C3H/HeJ mice. Here we show that this intermediate response to LPS is due to monoallelic expression of Tlr4. Allele usage is maintained during clonal expansion, a situation that resembles allelic exclusion. In contrast, Tlr4 is deleted on the recessive C57BL/10ScCr allele and all cells from F₁ mice from crosses of responder and C57BL/10ScCr mice express TLR4 protein. Thus, Tlr4 is an autosomal gene whose expression is regulated similarly to that of genes on the X chromosome.