Steric-blocking antisense, such as Morpholino oligos, can interfere with miRNA activity.

The first report that Morpholinos can block miRNAs is:
Kloosterman WP, Wienholds E, Ketting RF, Plasterk RH. Substrate requirements for let-7 function in the developing zebrafish embryo. Nucleic Acids Res. 2004 Dec 07;32(21):6284-91.
While the paper stated the knockdowns worked, no techniques or data were presented regarding the Morpholino knockdowns.

A recent paper describing the experimental techniques and results of a Morpholino-miRNA knockdown is:
Flynt AS, Li N, Thatcher EJ, Solnica-Krezel L, Patton JG. Zebrafish miR-214 modulates Hedgehog signaling to specify muscle cell fate. Nature Genetics 2007; 39: 259 - 263.

I have posted notes on the Flynt et al. paper on my blog,
http://blog.myspace.com/127542408

I'll post updates here as new papers are published using Morpholinos for miRNA knockdowns.

- Jon

People usually use LNA to knockdown miRNA, right? What are the advantages and disavantages of this morpholino oligo method?