Hi Nina,
siRNAs are gene specific, whatever gene you want to knockdown, just design a siRNA targeting that gene (mRNA, recently promoter) and you will have over 90% of chance of inactivating the gene. For example, you want to know which gene is lethal to a cell, you can design a siRNA (double-stranded short RNA) targeting that gene, and transfect the siRNA into your cell, then check if the expression of that gene is silenced and if there are resulted lethal phenotype changes.
Regarding essential genes, there is a recent paper in which the authors identified some essential genes by RNAi. It is a great paper which can be found here
http://www.ncbi.nlm.nih.gov/entrez/query.f...t_uids=11792820
Edited by pcrman, 22 September 2004 - 08:45 PM.