Eukaryotic gene expression is a complex process that can be controlled at the level of transcription, post-transcription, translation, or post-translation. In recent years there has been growing interest in understanding the role of the 3'-untranslated region (UTR) in post-transcriptional regulation. The 3'-UTR contains many regulatory sequences, including microRNA (miR) target sites and AU-rich elements (AREs). Although a relatively recent discovery, miRs have been shown to downregulate target gene expression and have important roles in regulating many cellular processes, including cellular growth. Cellular growth consists of two distinct states, proliferation and quiescence. The proliferative state consists of G1, S, G2, and M phases while quiescence is the G0 phase. In response to mitogenic stimuli, quiescent cells enter the proliferative cycle and may transit back to the quiescent state. Reentry into quiescence is essential to prevent aberrant proliferation as well as to protect the cellular life span. Cells that remain in quiescence for an extended period of time lose their ability to proliferate. It has been shown that the redox status of the cells may regulate quiescence and proliferative capacity since overexpression of SOD2 protects the proliferative capacity of quiescent cells. We hypothesized that the redox environment regulates proliferative capacity through miR expression and regulation of miR targets. Early results showed treatment with hydroxytyrosol (HT), an olive-derived catechol, was able to protect the proliferative capacity of quiescent normal human fibroblasts. HT was shown to use hydrogen peroxide and produce superoxide in a catechol-semiquinone-quinone redox cycle. Interestingly, HT also induced SOD2 expression. Further results from microRNA PCR arrays and Taqman PCR assays showed a significant decrease (4-fold) in miR-302 levels during quiescence compared to proliferating normal human fibroblasts, suggesting that miR-302 could regulate cellular growth states. Results from a Q-RT-PCR and dual luciferase-3'-UTR reporter assays identified ARID4a (AT-Rich Interacting Domain 4a, also known as RBP1) and CCL5 (C-C motif Ligand 1) as targets for miR-302. Ionizing radiation, that is well known to induce oxidative stress and delay cell cycle progression, decreased miR-302 levels, which was associated with an increase in its target mRNA levels, ARID4a and CCL5. Such an inverse correlation was also observed in cells treated with hydrogen peroxide, SOD2 overexpressing cells, and HT treated cells. Overexpression of miR-302 suppresses ARID4a and CCL5 mRNA levels, and increased the percentage of S-phase cells. These results identified miR-302 as a redox-sensitive regulator of ARID4a and CCL5 mRNAs as well as demonstrate a regulatory role of miR-302 during quiescence and proliferation.