PIWIL4 and breast cancer: This result, combined with that relative to PIWI proteins (Fig. 1A), indicates that all key components of the piRNA biogenesis and effector pathways identified in germline cells are present also in BC cells, a conclusion confirmed also for BT-549, HS578T, MDA-MB-231 and T47D BC cells, considering expression data from the NCI-60 [15] dataset (Supplementary Table S1B).