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Larvae of the body color mutant red blood (rb) of the silkworm, Bombyx mori, display reddish skin whose hemolymph becomes red in air, whereas hemolymphs of normal strains become black during melanization. The irregular coloring was assumed to result from an abnormal accumulation of 3-hydroxykynurenine. However, the gene responsible for the rb phenotype is not yet known. Here, we provide evidence that the rb gene corresponds to a novel bacterial-type kynureninase gene, BmKynu. Kynureninase (KYNU) hydrolyzes kynurenine and 3-hydroxykynurenine to anthranilic acid and 3-hydroxyanthranilic acid, respectively. KYNU has been identified in microorganisms and animals but not in insects. Therefore, BmKynu is the first KYNU gene observed in insects. Our results clearly showed that a point mutation (T102I) in BmKYNU of the rb strain led to a marked decrease in KYNU activity, presumably resulting in abnormal accumulation of 3-hydroxykynurenine. Additionally, linkage analysis indicated that no recombination between rb and BmKynu was detected. We conclude that T102I in BmKYNU causes the red body coloration in the rb strain. Our study proves that B. mori has a unique side branch in the kynurenine pathway, distinctly different from other insects.