Abstract 【Aim】 The chinese translucent (oc) mutant is one of the translucent mutants in the silkworm, Bombyx mori. Its responsible gene has been mapped at 40.8 cM on chromosome 5. The purpose of this study is to fine map and clone the candidate gene for oc mutation and to explore the molecular mechanism of the oc mutant. 【Methods】 A BC1 generation was got by hybridizing male F1 (the mutant strain oc×the wild-type strain Dz) with female oc. The markers were designed according to reference genomes, and those showing polymorphism between oc and Dz were used for the genetic analysis of 1 397 BC1 individuals. The candidate genes in the oc tightly linked region were analyzed by semi-quantitative RT-PCR and qPCR, and the target candidate gene was identified, cloned and sequenced. The cause of oc mutation was analyzed. 【Results】 The oc locus was mapped to the 234 kb region between markers M10 and M11 using 1 397 BC1 individuals and 11 polymorphic markers. There are five predicted protein-coding genes in this region. The expression analysis of the five predicted genes in the integument of ten strains of B. mori indicated that only BGIBMGA003572 was severely suppressed in the oc mutant as compared with in the wildtype strain. The sequence homology analysis indicated that the encoded protein of BGIBMGA003572 is homologous to the human monocarboxylate transporter 9, a possible uric acid transporter, which might be the candidate gene for oc mutation. The cloning and sequencing results of BGIBMGA003572 showed that five amino acids changed in the oc mutant as compared with the wild-type strain. 【Conclusion】 In this study, the oc locus was mapped to the 234 kb region by positional cloning technique, in which BGIBMGA003572 encoding a monocarboxylate transporter 9 might contribute to the phenotype of oc mutant.