PIWIL4 and cancer: We report here that Piwil2 and Piwil4 and other genes encoding known components of the piRNA biogenesis and effector pathways are expressed in human BC cells and that a specific set of piRNAs is expressed, some at a very high level, in normal and transformed mammary cell lines in vitro and tissues in vivo, demonstrating the presence of an active piRNA pathway in these cancers.