Gene Knocking-down
This technique allows the reduction of the expression of one or more of an organism. This can occur through genetic modification or by treatment with a reagent such as a short DNA or RNA oligonucleotide that has a sequence complementary to either gene or an mRNA transcript.
If the change in gene expression is caused by an oligonucleotide binding to an mRNA or temporarily binding to a gene, this leads to a temporary change in gene expression that does not modify the chromosomal DNA, and the result is referred to as a "transient knockdown".
In a transient knockdown, the binding of this oligonucleotide to the active gene or its transcripts causes decreased expression. Binding can occur through the blocking of transcription, the degradation of the mRNA transcript (e.g. siRNA or RNase-H dependent antisense), or through the blocking of either mRNA translation, pre-mRNA splicing sites, or nuclease cleavage sites used for maturation of other functional RNAs, including miRNA (e.g. by morpholino oligos).
The most direct use of transient knockdowns is for learning about a gene that has been sequenced, but has an unknown function. This experimental approach is known as reverse genetics. Transient knockdowns are often used in developmental biology because oligos can be injected into single-celled zygotes and will be present in the daughter cells of the injected cell.

RNA interference (RNAi) is a means of silencing genes by way of mRNA degradation. Gene knockdown by this method is achieved by introducing small double-stranded interfering RNAs (siRNA) into the cytoplasm. Once introduced into the cell, exogenous siRNAs are processed by RISC. The siRNA is complementary to the target mRNA to be silenced, and the RISC uses the siRNA as a template for locating the target mRNA. After the RISC localizes to the target mRNA, the RNA is cleaved by a ribonuclease.
RNAi is used for genetic functional analysis. The use of RNAi may be useful in identifying potential therapeutic targets, drug development, or other applications.

Gene Knocking-out
Gene knockout (KO) is a genetic technique in which one of an organism's genes is made inoperative. Knockout organisms are used to study gene function, usually by investigating the effect of gene loss. Heterozygous and homozygous Kos: in the former, only one allele is knocked out, in the latter both the two alleles are knocked out.

The directed creation of a KO begins in the test tube with a plasmid, a bacterial artificial chromosome or other DNA construct, and proceeding to cell culture. Cells are transfected with the DNA construct. Often the goal is to create a transgenic animal that has the altered gene.
If so, embryonic stem cells are genetically transformed and inserted into early embryos. Resulting animals with the genetic change in their germline cells can then often pass the gene knockout to future generations.

The construct is engineered to recombine with the target gene, which is done by incorporating sequences from the gene itself into the construct. Recombination then occurs in the region of that sequence within the gene, resulting in the insertion of a foreign sequence to disrupt the gene.
A conditional knockout allows gene deletion in a tissue or time specific manner. This is done by introducing loxP sites around the gene. These sequences will be introduced into the germ-line via the same mechanism as a knock-out. This germ-line can then be crossed to another germline containing Cre-recombinase which is a viral enzyme that can recognize these sequences, recombines them and deletes the gene flanked by these sites.
Because the DNA recombination is a rare event, the foreign sequence chosen for insertion usually includes a reporter. This enables easy selection of cells or individuals in which knockout was successful. Sometimes the DNA construct inserts into a chromosome without the desired homologous recombination with the target gene.

 (Ivana Venezia)