Thanks to RNA interference it's easier than ever to perform genetic knockouts in cultured cells and live animals. The idea is simple enough: deliver a short interfering RNA (siRNA) for the desired mRNA, wait a few days, and see what happens. In theory, the siRNAs (short double-stranded RNAs homologous to a region of an mRNA) will program the RNA-induced silencing complex to target the desired transcript for degradation.
In practice, however, it takes time, and a lot of optimization, to get the process working. This is less true for researchers whose target genes correspond to precloned interference constructs such as those available from Cold Spring Harbor Laboratory (http://codex.cshl.edu) or commercial vendors such as Sigma-Aldrich, Open Biosystems, and System Biosciences. You might also get lucky and be able to purchase ready-to-use siRNAs from Ambion, Qiagen, or Dharmacon. But suppose you need to go from gene to gene knockout completely from scratch? ...
