AF5q31 (also called MCEF) was identified by its involvement in chromosomal translocation with the gene MLL (mixed lineage leukemia), which is associated with infant acute lymphoblastic leukemia. Several potential roles have been proposed for AF5q31 and other family genes, but the specific requirements of AF5q31 during development remain unclear. Here, we show that AF5q31 is essential for spermatogenesis. Although most AF5q31-deficient mice died in utero and neonatally with impaired embryonic development and shrunken alveoli, respectively, 13% of AF5q31-deficient mice thrived as wild-type mice did. However, the male mice were sterile with azoospermia. Histological examinations revealed the arrest of germ cell development at the stage of spermiogenesis, and virtually no spermatozoa were seen in the epididymis. AF5q31 was found to be preferentially expressed in Sertoli cells. Furthermore, mutant mice displayed severely impaired expression of protamine 1, protamine 2, and transition protein 2, which are indispensable to compact the haploid genome within the sperm head, and an increase of apoptotic cells in seminiferous tubules. Thus, AF5q31 seems to function as a transcriptional regulator in testicular somatic cells and is essential for male germ cell differentiation and survival. These results may have clinical implications in the understanding of human male infertility.Chromosomal translocation is one of the common pathogenic mechanisms in various human malignancies, particularly in leukemias and lymphomas, and genes located at the breakpoints are involved in disease pathogenesis (21,59,60). The mixed lineage leukemia gene MLL (also called HRX, HTRX, and ALL-1) is frequently targeted by chromosomal rearrangements and is associated with clinically aggressive lymphoid and myeloid leukemias which are particularly prevalent in infant leukemias and treatment-related secondary leukemias (2,18,24,64). MLL located on 11q23 is a human homologue of Drosophila trithorax, has a SET domain that normally functions as histone methyltransferase, and is assembled into a supermultiprotein complex with additional chromatin-remodeling components (45,50,70). Importantly, most of the leukemic variants of MLL lack the SET domain (7). In Drosophila, genetic evidence suggests that Trithorax controls the expression of homeobox (Hox) genes and regulates embryogenesis (39,44,47). In MLL-deficient mice, Hox gene expression initiates normally but is not maintained after 9.5 days postcoitus (dpc), demonstrating the importance of MLL in the maintenance of Hox gene expression (72,73). Hox genes also play an important role in hematopoietic differentiation, and their expression levels are upregulated in the human leukemias carrying MLL rearrangements (1). An unusual feature of MLL fusion proteins is the large number and diversity of heterologous proteins that fuse with MLL. To date, the MLL locus has been found to be translocated to approximately 40 different genetic loci and at least 30 of the partner genes have been characterized (13,31). The fun...