Table 4.

Cancer-related PIWI Proteins

PIWI Proteins	Cancer	Expression	Function	References
PIWIL1/HIWI	Seminoma Cancer Colorectal cancer Breast cancer Gastric cancer Lung cancer Ovarian cancer Glioma Sarcomas Endometrial cancer Hepatocellular carcinoma Cervix cancer	Increased Increased Increased Increased Increased Increased Increased Increased Decreased Decreased Increased	Encourage cell propagation Inhibit migration and invasion Increase cell proliferation and stemness Encourage cell proliferation CSC self-renewa1 Migration and invasion Increase cell propagation Genomic instability Related to DNA hypermethylation Reduce cell proliferation and migration Migration and invasion	[61,124,170,182–190,190–192]
PIWIL2/HILI	Colon cancer Liver cancer Breast cancer Gastric cancer Ovarian cancer Cervix cancer Esophageal cancer Soft-tissue sarcoma	Increased Increased Increased Increased Increased Increased Increased Decreased	Proliferation, migration, and invasion Apoptosis inhibition Proliferation, apoptosis inhibition, migration, and invasion Related to poor prognosis Genomic instability Inhibit apoptosis Related to poor prognosis Reduce cell proliferation and increase apoptosis	[193, Z. 194] [J. H. 67, 195] [J. H. 196, H. 160] [175,185,190,197,198]
PIWIL3/HIWI3	Breast cancer Gastric cancer	Increased Increased	Prognosis relevance Proliferation, migration and invasion	[143,199]
PIWIL4/HIWI2	Cervical cancer Lung cancer Colon cancer Breast cancer	Increased Increased Increased Increased	Impair apoptosis, promote invasion Proliferation Migration and invasion Prognosis relevance	[D.-W. 200, X. [201] [L. 202] [143]