The transformer-2(tra-2) gene plays a key role in the regulatory hierarchy of sexual differentiation in somatic tissues and in the germline of Drosophila melanogaster.In this study,sequences and expression profiles of tra-2 in the Chinese mitten crab Eriocheir sinensis were characterized.Four tra-2 isoforms,designated as Estra-2a,Estra-2b,Estra-2c,and Estra-2d,were isolated.They all contained an RNA-recognition motif(RRM) and a linker region,which shared high similarity with other reported tra-2s.Sequence analysis revealed that Estra-2a,Estra-2b and Estra-2c are encoded by the same genomic locus and are generated by alternative splicing of the pre-mRNA.Compared with the other three isoforms,Estra-2d lacks the RS2 domain.Quantitative real-time PCR showed that all four isoforms were highly expressed in the fertilized egg,and in the 2-4 cell and blastula stages compared with larval stages(P<0.01),suggesting their maternal origin in early embryonic developmental stages.Notably,Estra-2a was highly expressed in male somatic tissues,while Estra-2c was significantly highly expressed in the ovary.These results suggest that Estra-2c is involved in sexual differentiation of the Chinese mitten crab.Our findings provide basic information for further functional studies of the tra-2 gene/protein in this species. The transformer-2 (tra-2) gene plays a key role in the regulatory hierarchy of sexual differentiation in somatic tissues and in the germline of Drosophila melanogaster. In this study, sequences and expression profiles of tra-2 in the Chinese mitten crab Eriocheir The all-contained RNA-recognition motif (RRM) and a linker region, which shared high similarity with other reported tra-2s.Sequence analysis revealed that Estra-2a, Estra-2b and Estra-2c are encoded by the same genomic locus and are generated by alternative splicing of the pre-mRNA. Compared with the other three isoforms, Estra-2d lacks the RS2 domain. Quantitative real-time PCR showed that all four isoforms were highly expressed in the fertilized egg, and in the 2-4 cell and blastula stages compared with larval stages (P <0.01), suggesting their maternal origin in early embryonic developmental stages. Notably, Estra-2a was highly expressed in male somatic tissues, while Estra-2c was significantly highly expressed in the ovary.These results suggest that Estra-2c is involved in sexual differentiation of the Chinese mitten crab.Our findings provide basic information for further functional studies of the tra- 2 gene / protein in this species.