The protein Dicer is required for microRNA (miRNA) biogenesis. Dicer-deficient cells therefore lack almost all mature, functional miRNAs. We investigated the role of miRNAs in regulation of gene expression in mouse ES cells by analysing gene expression in either wildtype or Dicer-deficient cells grown either under asynchronous growth conditions, or cells in the early G1 phase of the cell cycle. RNA was extracted from three biological replicates of both wildtype and Dicer-deficient ES cells grown under asynchronous culture conditions. To obtain cells in early G1, populations (in triplicate for both cell types) were cultured in 20ng/ml demecolcine for 4 hours, then mitotic cells were isolated using the mitotic shake-off technique (Terasima & Tolmach, Experimental Cell Research (30) 1963). Mitotic cells were replated in demecolcine-free media for 2 hours, allowing them to progress in a highly synchronised manner into early G1.