Sertoli cells provide the cytoarchitectural support and microenvironment necessary for the process of spermatogenesis and have a central role in male sex determination. Characterization of Sertoli cell specific gene products provides insight into the unique functions of this testicular cell type. The current study reports the identification of a novel Sertoli cell gene product that is termed Sertoli cell specific gene (SERT). The SERT cDNA sequence shows no homology to any of the known gene sequences in the GenBank database. Some homology was found with short sequences in the expressed sequence tag (EST) database. A motif analysis demonstrates a signature to an RNA binding protein sex lethal (sx1) involved in Drosophila sex determination. The expression pattern of SERT was examined with reverse transcription polymerase chain reaction (RT-PCR) and Northern blot analysis. SERT expression was found to be specific to the testis and absent in other tissues examined. In the testis, the approximate 1.5-kb SERT transcript was localized to the Sertoli cells. A 1.1-kb spliced form of SERT was also identified by reverse transcription polymerase chain reaction analysis. The genomic structure analysis demonstrates the presence of at least three exons with 85% sequence homology between mouse and rat sequences. Treatment of cultured Sertoli cells with follicle stimulating hormone (FSH) significantly increased the expression of SERT mRNA levels. The potential role of SERT in Sertoli cell function was investigated with the use of a transferrin promoter reporter construct. Transferrin expression is a marker of Sertoli cell differentiated function. An antisense oligonucleotide to SERT significantly inhibited FSH and cAMP induced transferrin promoter activation, while control oligonucleotides had no effect. In summary, a novel gene product expressed primarily by Sertoli cells, SERT, was identified with a potential RNA binding protein motif similar to sex lethal that appears to have a role in maintaining hormone (e.g. FSH) actions in Sertoli cells.