Knockout

Gene knockout is a genetic technique used to discover functions of specific sequenced genes or DNA regions. In this technique the function of the gene of interest is totally abolished, made inoperative. The phenotype of the knocked-out organism is then compared to a wildtype.
More than one gene can be simultaneously knocked out in the organism. If two genes are knocked out, we talk about a double knockout (DKO). Knockout of three or four genes results in forming a triple knockout (TKO) or a quadruple knockout (QKO). 
An opposite to genetic knock-out is the knock-in. Knock-in is a genetic engineering technique which involves insertion of a gene cDNA into an host organism. 
One od the most commonly used systems for genetic knockout is the Cre-Lox system (Lambert et al., 2007). This system also allows a conditional knockout – in a tissue or a time specific manner by adding the Cre-recombinase. The Cre-lox cystem is mainly used for acquiring knock-out mice. In tissue cultures, gene silencing is usually provided by the usage of miRNA (micro-silencing RNAs) (Fabian and Sonenberg, 2012). MicroRNAs (miRNAs) are capable of post-transcritionally regulating protein accumulation. This repression is mediated by recruitment of a miRNA-induced silencing complex (miRISC) to target mRNAs, which are then degraded. 
Knockout organisms are used for genetic screening, drug development or targeting specific biological processes.