Schematic representation of the design miR-181a sponge and the control sponge. miRNA antisense binding sites (MBS) contain central mismatch at position 9-12 of the miRNA sequence (bulge). For the control sponge, a scrambled sequence at positions 2-8 was generated. The two MBS are separated by a short 4-6 nt sequence spacer. Bold flanking letters indicate overhangs compatible with the restriction endonuclease SanDI.