Argonaute family proteins are closely linked to small non-coding RNAs (siRNAs, microRNAs, piRNAs). With their functional domains, Argonaute proteins can bind small non-coding RNAs and control protein synthesis, finally affecting mRNA stability, namely RNA interference (RNAi). Moreover, they also participate in the production of Piwi-interacting RNAs (piRNAs). There are 8 members in the human Argonaute family. Using quantitative RT-PCR (qRT-PCR), the expression levels of 4 Ago genes (Ago1～Ago4) were assayed. Experimental results indicated that these genes were ubiquitously expressed in many cell lines. Then it was investigated whether Argonaute subfamily proteins could influence cell cycle regulation by knocking down their expressions in human breast adenocarcinoma MCF-7 cells and cervical carcinoma HeLa cells. The lack of Ago expression resulted in decreased cellular proliferation activity. Further investigation revealed that the cell cycle was delayed at the G0/G1 phases with the especially remarkable arrest occurring in the cases of Ago1 (P < 0.01) and Ago4 (P < 0.05), but without apoptosis. It suggests that Argonaute proteins may function in the cell cycle progression through a possible pathway that does not require small RNAs. The mechanisms underlying this phenomenon are still a puzzle for us.