PIR and breast cancer: Several cell-free piRNAs have been reviewed previously [311] and implicated in malignant diseases: in colorectal cancer (e.g., piR-5937; piR-54265), breast cancer (e.g., piR-651, piR-17458), gastric cancer (e.g., piR-019308, piR-004918), lung cancer (e.g., piR-hsa-164586, piR-hsa-5444), and cholangiocarcinoma (e.g., piR-14090389) [311,312,313,314,315,316,317,318,319].