The Argonaute proteins, which are the direct partners of the small RNAs involved in RNA interference mechanisms, can be divided into two subfamilies, the Argonautes and the Piwis. In animals, the Argonaute subfamily binds 21-22 nucleotide small interfering RNAs (siRNAs) and microRNAs (miRNAs), which direct cleavage and translational inhibition of their target RNAs respectively. The partners of the Piwi proteins are 24-30-nucleotide small RNAs called Piwi-interacting RNAs or piRNAs. In Drosophila, Piwi proteins and piRNAs protect the genome of the germline against selfish elements. Recent studies suggest that this function is conserved in mammals.