The article "PIWI-interacting RNAs: small RNAs with big functions" by Ozata et al. (2018) reviews the role of PIWI-interacting RNAs (piRNAs) in animal biology. PiRNAs are a class of small RNAs that play crucial roles in silencing transposable elements, regulating gene expression, and defending against viral infections. They guide PIWI proteins to target RNAs, promote heterochromatin assembly, and methylate DNA. The piRNA pathway provides both adaptive and innate immunity against rapidly evolving viruses and transposons. PiRNAs silence transposons in the germline of most animals, while somatic piRNA functions have evolved differently across species. The article discusses the biogenesis of piRNAs, including the unique mechanisms in different animals, and their diverse functions, such as transposon silencing, viral defense, and regulation of host gene expression. It highlights the conservation and divergence of piRNA pathway proteins and the evolution of piRNA-producing loci. The review also explores the challenges faced by germline genomes in different animals and the role of piRNAs in maintaining genome stability.The article "PIWI-interacting RNAs: small RNAs with big functions" by Ozata et al. (2018) reviews the role of PIWI-interacting RNAs (piRNAs) in animal biology. PiRNAs are a class of small RNAs that play crucial roles in silencing transposable elements, regulating gene expression, and defending against viral infections. They guide PIWI proteins to target RNAs, promote heterochromatin assembly, and methylate DNA. The piRNA pathway provides both adaptive and innate immunity against rapidly evolving viruses and transposons. PiRNAs silence transposons in the germline of most animals, while somatic piRNA functions have evolved differently across species. The article discusses the biogenesis of piRNAs, including the unique mechanisms in different animals, and their diverse functions, such as transposon silencing, viral defense, and regulation of host gene expression. It highlights the conservation and divergence of piRNA pathway proteins and the evolution of piRNA-producing loci. The review also explores the challenges faced by germline genomes in different animals and the role of piRNAs in maintaining genome stability.