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The mutant of non-molting in the 2nd instar(nm2)is a recently discovered mutant of Bombyx mori.The mutant can not molt and exuviate and died successively in premolting of 2nd instar.In this study,two-dimensional gel electrophoresis(2-DE)was performed to screen the differential expression of epidermis proteins in pre-molting larvae of 2nd instar between the wild-type and nm2 mutant.Interestingly,a cysteine proteinase-like(BmCP-like)protein in nm2 was significantly higher than that of the wild-type.The transcription profiles of BmCP-like gene were investigated by quantitative real-time PCR(qRT-PCR),and the result revealed that BmCP-like mRNA was remarkably higher in nm2 than that of the wild-type.The transcription level of BmCP-like was high in the epidermis while low in the midgut and hemocytes,and fluctuate with development,while the highest in the newly molted larvae of 3rd and lowest in the pre-molting of the 1st and 2nd instar.The body of injected BmCP-like RNAi of 2nd larvae formed a dark spots around the injection place.These results suggested the BmCP-like gene play a key role in the degradation of the cuticle and epidermis layer during molting of 1st and 2nd instar silkworm.Furthermore,the ORF of BmCP-like gene in nm2 was the same to the wild-type.These studies give us a hint that BmCP-like gene maybe not the major gene responsible for nm2,but BmCP-like gene might participate in the immune systems of silkworm,and the upregulation of BmCP-like transcription in the nm2 mutant might be induced by the disadvantages that limit the growth and development of silkworm in order to survive.