ATP-dependent RNA helicase required during spermatogenesis (PubMed:10920202, PubMed:21034600). Required to repress transposable elements and preventing their mobilization, which is essential for the germline integrity (By similarity). Acts via the piRNA metabolic process, which mediates the repression of transposable elements during meiosis by forming complexes composed of piRNAs and Piwi proteins and governs the methylation and subsequent repression of transposons (By similarity). Involved in the secondary piRNAs metabolic process, the production of piRNAs in fetal male germ cells through a ping-pong amplification cycle (By similarity). Required for PIWIL2 slicing-triggered piRNA biogenesis: helicase activity enables utilization of one of the slice cleavage fragments generated by PIWIL2 and processing these pre-piRNAs into piRNAs (By similarity). ATP + H2O = ADP + H(+) + phosphate Found in a mRNP complex, at least composed of TDRD1, TDRD6, TDRD7 and DDX4 (By similarity). Interacts with RANBP9 (PubMed:27622290). Interacts with RANBP10. Interacts with PIWIL2 and MAEL. Interacts with ARNTL/BMAL1 and CLOCK. Interacts with Tex19.1 and, probably, Tex19.2 (By similarity). Component of the meiotic nuage, also named P granule, a germ-cell-specific organelle required to repress transposon activity during meiosis. Expressed only in ovary and testis. Expressed in migratory primordial germ cells in the region of the gonadal ridge in both sexes. Belongs to the DEAD box helicase family. DDX4/VASA subfamily.