Phospholipid hydroperoxide glutathione peroxidase (GPx4) is an intracellular antioxidant enzyme that directly reduces peroxidized phospholipids. GPx4 is strongly expressed in the mitochondria of testis and spermatozoa. We previously found a significant decrease in the expression of GPx4 in spermatozoa from 30% of infertile human males diagnosed with oligoasthenozoospermia (Imai, H., Suzuki, K., Ishizaka, K., Ichinose, S., Oshima, H., Okayasu, I., Emoto, K., Umeda, M., and Nakagawa, Y. (2001) Biol. Reprod. 64, 674 -683). To clarify whether defective GPx4 in spermatocytes causes male infertility, we established spermatocyte-specific GPx4 knock-out mice using a CreloxP system. All the spermatocyte-specific GPx4 knock-out male mice were found to be infertile despite normal plug formation after mating and displayed a significant decrease in the number of spermatozoa. Isolated epididymal GPx4-null spermatozoa could not fertilize oocytes in vitro. These spermatozoa showed significant reductions of forward motility and the mitochondrial membrane potential. These impairments were accompanied by the structural abnormality, such as a hairpin-like flagella bend at the midpiece and swelling of mitochondria in the spermatozoa. These results demonstrate that the depletion of GPx4 in spermatocytes causes severe abnormalities in spermatozoa. This may be one of the causes of male infertility in mice and humans.A frequent cause of male infertility is defective sperm function, which is the main problem for close to a quarter of couples who attend infertility clinics (1-4). Considerable efforts are now focused on the identifying ultrastructural and/or molecular defects in the spermatozoa or seminal plasma to develop solutions to various types of male infertility.Phospholipid hydroperoxide glutathione peroxidase (GPx4) 2 is an intracellular selenoprotein that directly reduces peroxidized phospholipids produced in cell membranes (5). The GPx4 gene has a complex intron/exon structure (6, 7). Three different transcripts of GPx4 exist, differing in their 5Ј extension and coding for a cytosolic protein (non-mitochondrial GPx4), a mitochondrial protein (mitochondrial GPx4), and a nuclear protein (nucleolar GPx4), respectively (6, 7). After cleavage of the N-terminal mitochondrial import sequence of mitochondrial GPx4, the mature protein becomes identical to the 20-kDa non-mitochondrial GPx4 (8, 9). Nuclear GPx4 was recently identified as a sperm nucleus-specific 34-kDa selenoprotein (called snGPx, for sperm nucleus-specific glutathione peroxidase) (10). It is formed by use of an alternative promoter and start codon localized in the first intron of the GPx4 gene (7, 10, 11). We previously reported that 34-kDa GPx4 localized in nucleoli in several cell lines by using an N-terminal nucleolar import signal (11). We call hereafter nuclear GPx4 nucleolar GPx4, because non-mitochondrial 20-kDa GPx4 exists both in cytosol and in the nucleus (12). Expression of three types of GPx4 is induced significantly in testis during spermatogenesis, especiall...