Defining the functions of PIWI-interacting RNAs

In the animal germ line, PIWI-interacting RNAs (piRNAs) silence transposons and may also control gene expression. Distinct, but not incompatible, models have been proposed for how piRNAs regulate genes. To promote discussion in the small RNA community of how best to test these models, we propose criteria for validating piRNA function. Wu and Zamore discuss models of piRNA function and suggest experimental criteria to be followed in order to validate them.