Mirk regulates G0/G1 to S phase cell cycle via MAPK/ERK signaling. To investigate this possibility that Mirk expression is associated with activated ERK1/2, the human cancer cells treated with 20 nM siRNAs alone or in combination with a range of concentrations of U0126 for 72 h were collected, and followed by (A) the protein expressions of Mirk (69 and 71 Kda), ERK1/2 and P-ERK1/2 (42/44 kDa), C-Raf and P-C-Raf (74 kDa), as well as alterations of cyclin D1 (36 Kda) and p27kip1 (27 kDa) in H292 or OVCAR3 cells were measured by western blot analyses. Equal loading and transfer were shown by repeat probing with β-actin (42 kDa); (B) H292 cells stained with propidium iodide were subjected to flow cytometry analysis; and (C) H292 growth rate was measured by MTT assay. *, P < 0.05; **, P < 0.01 compared with control. siRNA, small interfering RNA; MAPK, mitogenic-activated protein kinase; ERK, extracellular signal-regulated kinase. Ctrl, no siRNA; Ctrl si, control siRNA; Mirk si, Mirk siRNA.