In about half the cases of involuntary childlessness,a male infertility factor is involved.The PIWI-LIKE genes,a subclade of the Argonaute protein family,are involved in RNA silencing and transposon control in the germline.Knockout of murine Piwi-like 1 and 2 homologs results in complete infertility in males.The aim of this study was to analyze whether the mRNA expression of human PIWI-LIKE 1-4 genes is altered in ejaculated spermatozoa of men with impaired sperm characteristics.Ninety male participants were included in the study,among which 47 were with normozoospermia,36 with impaired semen characteristics according to the World Health Organization (WHO) manual,5th edition,and 7 with azoospermia serving as negative control for the PIWI-LIKE 1-4 mRNA expression in somatic cells in the ejaculate.PIWI-LIKE 1-4 mRNA expression in the ejaculated spermatozoa of the participants was measured by quantitative real-time PCR.In nonazoospermic men,PIWI-LIKE 1-4 mRNA was measurable in ejaculated spermatozoa in different proportions.PIWI-LIKE 1 (100.0％) and PIWI-LIKE 2 (49.4％) were more frequently expressed than PIWI-LIKE 3 (9.6％) and PIWI-LIKE 4 (15.7％).Furthermore,a decreased PIWI-LIKE 2 mRNA expression showed a significant correlation with a decreased sperm count (P=0.022) and an increased PIWI-LIKE 1 mRNA expression with a decreased progressive motility (P =0.048).PIWI-LIKE 1 and PIWI-LIKE 2 mRNA expression exhibited a significant association with impaired sperm characteristics and may be a useful candidate for the evaluation of the impact of PIWI-LIKE 1-4 mRNA expression on male infertility.