Among these piRNAs, abnormal expression of pir-651 and pir-823 is the most widely associated with various cancer types.406 It has been proved that in gastric cancer, the expression of piR-651 is significantly higher than in adjacent normal tissue,404 while the expression of piR-823 is markedly reduced.407 Concurrently, the expression of PIWIL1 is upregulated in gastric cancer cells. This evidence concerns the gene PIWIL1 and cancer.