Required during spermiogenesis to participate in the repression transposable elements and prevent their mobilization, which is essential for the germline integrity. Probably 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 govern the methylation and subsequent repression of transposons. Required for chromatoid body (CB) assembly. Localizes to chromatoid body (CB) and pi-body (also called intermitochondrial cementin), 2 cytoplasmic ribonucleoprotein granules involved in RNA processing for spermatogenesis. Gonad-specific. Mainly expressed in testis. Present at low level in ovary (at protein level). Specifically expressed in germ cells of the developing testis, starting from 12.5 dpc. The protein is detected as early as 13.5 dpc in embryonic testes, but expression declines around the perinatal period and then is restored at around 2 weeks after birth. Also detected in the embryonic ovary at a low level. In adult testis, mainly present in the spermatocytes from the pachytene stage onward (at protein level). Male sterility because of spermiogenic arrest at the round spermatid stage, with occasional failure in meiotic prophase. Effects are due to demethylation and subsequent derepression of transposable elements: germ cells fail to repress LINE-1 (L1) retrotransposons with DNA-demethylated promoters. Defects in chromatoid body (CB) and pi-body assembly are also observed. Interestingly, Tdrd5-deficient round spermatids injected into oocytes contribute to fertile offspring, showing that acquisition of a functional haploid genome may be uncoupled from Tdrd5 function. Belongs to the TDRD5 family.