Summary: Targeted gene knockdown using small interfering RNA (siRNA) or antisense oligonucleotide has been important technology for studying gene function. Gene knockdown leads to reduction in mRNA and subsequently protein expression. It can be often verified at mRNA level by Northern blot or quantitative RT-PCR. However, a decrease in the amount of a specific mRNA does not typically correlate well with protein levels present in the cell. Gene knockdown can be also measured at the protein level with Western blot.Western blot analysis is the most comprehensive way of showing that expression of the target gene has been down regulated. However this method, while sensitive, often lacks the ability to discriminate between samples in which the differences in protein levels are minimal. It is also limited in its application to high-throughput analysis.