RA175/TSLC1/SynCAM/IGSF4A (RA175), a member of the immunoglobulin superfamily with Ca 2؉ -independent homophilic trans-cell adhesion activity, participates in synaptic and epithelial cell junctions. To clarify the biological function of RA175, we disrupted the mouse Igsf4a (Ra175/Tslc1/SynCam/Igsf4a Ra175) gene. Male mice lacking both alleles of Ra175 (Ra175 ؊/؊ ) were infertile and showed oligo-astheno-teratozoospermia; almost no mature motile spermatozoa were found in the epididymis. Heterozygous males and females and homozygous null females were fertile and had no overt developmental defects. RA175 was mainly expressed on the cell junction of spermatocytes, elongating and elongated spermatids (steps 9 to 15) in wild-type testes; the RA175 expression was restricted to the distal site (tail side) but not to the proximal site (head side) in elongated spermatids. In Ra175 ؊/؊ testes, elongated and mature spermatids (steps 13 to 16) were almost undetectable; round spermatids were morphologically normal, but elongating spermatids (steps 9 to 12) failed to mature further and to translocate to the adluminal surface. The remaining elongating spermatids at improper positions were finally phagocytosed by Sertoli cells. Furthermore, undifferentiated and abnormal spermatids exfoliated into the tubular lumen from adluminal surfaces. Thus, RA175-based cell junction is necessary for retaining elongating spermatids in the invagination of Sertoli cells for their maturation and translocation to the adluminal surface for timely release.RA175/TSLC1/SynCAM/IGSF4A (RA175) is a new member of the immunoglobulin superfamily that has three immunoglobulin domains in its extracellular region and EYFI, a type II PDZ (postsynaptic density 95/disk large/zonula occluders-1)-binding domain, in its C-terminal intracellular region and has Ca 2ϩ -independent homophilic trans-cell adhesion activity (10). RA175 was at first isolated as one of the genes preferentially expressed during the neuronal differentiation of P19 mouse embryonal carcinoma (P19 EC) cells induced by retinoic acid (9, 21) and is localized in the mouse developing nervous system and epithelium of various organs (2,10,11,29). In the nervous system, RA175 is designated the synaptic cell adhesion molecule (SynCAM) and has been suggested to be involved in the formation of functional synapses (2). The localization of RA175 also suggests its biological role in the migration of neurons and the fasciculation of axons (11). In the developing lung epithelium, RA175 is localized in the celladherent region of the basolateral membrane in the polarized cells lining the lumen (10). Defects in IGSF4A/tumor suppressor gene in lung cancer 1 (TSLC1), an orthologue of mouse Igsf4a (Ra175), promote the metastasis of lung carcinoma cells (18). However, little is known about its real biological function during organ development. To clarify the biological function of RA175, we disrupted the murine Ra175 gene. Male mice lacking both alleles of Ra175 (Ra175 Ϫ/Ϫ ) were infertile, whereas heterozygous males...