ABSTRACT: TSLC1/IGSF4, an immunoglobulin superfamily molecule, is predominantly expressed in the brain, lung, and testis and plays important roles in epithelial cell adhesion, cancer invasion, and synapse formation. We generated Tslc1/Igsf4-deficient mice by disrupting exon 1 of the gene and found that Tslc1-/- mice were born with the expected Mendelian ratio but Tslc1-/- male mice were infertile. In adult Tslc1-/- mice of 11-week of age, the weight of the testis was 90% of that of the Tslc1+/+ mice, and the number of sperm in the semen was approximately 0.01% of it. Histological analysis revealed that the round spermatids and the pachytene spermatocytes failed to attach to the Sertoli cells in the seminiferous tubules and sloughed off into the lumen with apoptosis in the Tslc1-/- mice. On the other hand, the spermatogonia and the Sertoli cells, as well as the interstitial cells, were essentially unaffected. In the Tslc1+/+ mice, TSLC1/IGSF4 expression was observed in the spermatogenic cells from the intermediate spermatogonia to the early pachyten spermatocytes and from step-7 or later spermatids. These findings suggest that TSLC1/IGSF4 expression is indispensable for the adhesion of spermatocytes and spermatids to Sertoli cells and for their normal differentiation to mature spermatozoa. Experiment Overall Design: The protocol used for the sample preparation and microarray processing is available from Affymetrix (Santa Clara, CA, USA). Briefly, 3 ïg purified RNA, extracted from the testis from Tslc1+/+ and Tslc1-/- mice of 16-week-old, was reverse-transcribed with Superscript II reverse transcriptase (Invitrogen) using the primer T7-dT24 containing a T7 RNA polymerase promoter. After a second strand of cDNA was synthesized using RNase H, Escherichia coli DNA polymerase, and E. coli DNA ligase, in vitro transcription was carried out on the cDNA to produce biotin-labeled cRNA with a MEGAscript High Yield Transcription Kit (Ambion, Austin, TX, USA) as recommended by the manufacturer. After the cRNA was linearly amplified with T7 polymerase, the biotinylated cRNA was cleaned with an RNeasy Mini Column (Qiagen), fragmented to 50 to 200 nucleotides, and then hybridized to Mouse Genome U74A ver.2 arrays (Affymetrix). The stained microarray was scanned with a GeneArray Scanner (Affymetrix), and the signal was calculated with Affymetrix software, Microarray Suite 5.0. All of the data were scaled with the global scaling method to adjust the target intensity to 1,000.