Many cancers are characterised by global reductions in microRNA (miRNA) expression when compared to normal tissue. The mechanisms underlying this global reduction in miRNA expression are unknown, though some evidence exists for post-transcriptional control via the influence of p53 on Drosha, and disruption of miRNA processing. Hypoxia and increased expression of hypoxia inducible factor (HIF-1) are common features in many tumours, and can correlate both with tumour aggression and patient outcome. In a microarray analysis of MCF7 cells exposed to hypoxia (1%, 16 hours), we had observed a consistent reduction in mRNA expression of the key miRNA biogenesis protein Dicer which cleaves the pre-miRNA hairpin to generate a mature miRNA. Therefore, in this study we examined the hypoxic regulation of Dicer mRNA and protein. Breast cancer cells (MCF7, SKBR3) and colorectal adenocarcinoma cells (HT29) were exposed to hypoxic conditions (0.1%-5% for 8-48 hours) or the HIF hydroxylase inhibitor, dimethyloxalylglycine (DMOG) (1 mM for 16-48 hours). The mRNA levels were assayed by RT PCR and protein levels assayed by Western blotting. Significant reductions (P<0.05) of Dicer mRNA and protein levels were observed in MCF7, SKBR3 and HT29 cells after exposure to hypoxia and DMOG. The most significant down regulation of mRNA (2.5-fold P=0.008) and protein (3-fold p=0.04.) were seen at 0.1% O2 for 48 hours in MCF7 cells. These results suggest that in some cancer cells Dicer levels may be reduced via HIF-1 in hypoxia. We have also seen a hypoxic repression of other mRNAs encoding proteins with important roles in microRNA biogenesis and function (Drosha (3.2-fold P=0.05), AGO2 (5.5-fold P=0.002) and TARBP2 (4-fold, P=0.002). We are currently examining the HIF dependence of this process and the effects of hypoxia on microRNA biogenesis and function.