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)

KNOCKDOWN
It is a technique by which the expression of a gene is reduced. This reduction can result from a DNA modification or from an oligonucleotide binding either to the mRNA or to the gene. In this case the expression change is temporary, so we talk about transient knockdown.
One of the techniques allowing the transient knockdown of a gene consists in the use of Morpholino oligomers. They have become a standard knockdown tool in animal embryonic systems, so Morpholino oligos are often used to investigate the role of a specific mRNA transcript in an embryo. Its molecular structure has DNA bases attached to a backbone of methylenemorpholine rings linked through phosphorodiamidate groups. Morpholinos block access of other molecules to small (~25 base) specific sequences of the base-pairing surfaces of ribonucleic acid (RNA). Because of their completely unnatural backbones, Morpholinos are not recognized by cellular proteins. Nucleases do not degrade Morpholinos, nor are they degraded in serum or in cells. There are no published reports of Morpholinos activating toll-like receptors or innate immune responses such as interferon induction or the NF-κB-mediated inflammation response. Morpholinos are not known to modify DNA methylation.
Morpholinos do not trigger the degradation of their target RNA molecules, unlike many antisense structural types (e.g., phosphorothioates, siRNA). Instead, Morpholinos act by "steric blocking", binding to a target sequence within an RNA, inhibiting molecules that might otherwise interact with the RNA.
Morpholinos can also modify the splicing of pre-mRNA or inhibit the maturation and activity of miRNA.
Blocking translation
Bound to the 5'-untranslated region of messenger RNA (mRNA), Morpholinos can interfere with progression of the ribosomal initiation complex from the 5' cap to the start codon. This prevents translation of the coding region of the targeted transcript (called "knocking down" gene expression). This is useful experimentally when an investigator wishes to know the function of a particular protein; Morpholinos provide a convenient means of knocking down expression of the protein and learning how that knockdown changes the cells or organism.
Modifying pre-mRNA splicing
Morpholinos can interfere with pre-mRNA processing steps either by preventing splice-directing small nuclear ribonucleoproteins (snRNP) complexes from binding to their targets at the borders of introns on a strand of pre-mRNA, or by blocking the nucleophilic adenine base and preventing it from forming the splice lariat structure, or by interfering with the binding of splice regulatory proteins such as splice silencers and splice enhancers. Preventing the binding of snRNP U1 (at the donor site) or U2/U5 (at the polypyrimidine moiety and acceptor site) can cause modified splicing, commonly excluding exons from the mature mRNA. Targeting some splice targets results in intron inclusions, while activation of cryptic splice sites can lead to partial inclusions or exclusions. Targets of U11/U12 snRNPs can also be blocked. Splice modification can be conveniently assayed by reverse-transcriptase polymerase chain reaction (RT-PCR) and is seen as a band shift after gel electrophoresis of RT-PCR products.
KNOCKOUT
A variation of the traditional gene knockout is the Conditional gene knockout. 
Conditional gene knockout is a technique used to eliminate a specific gene in a certain tissue, such as the liver. This technique is useful to study the role of individual genes in living organisms. It differs from traditional gene knockout because it targets specific genes at specific times rather than being deleted from beginning of life. Using the conditional gene knockout technique eliminates many of the side effects from traditional gene knockout. In traditional gene knockout, embryonic death from a gene mutation can occur, and this prevents scientists from studying the gene in adults. Some tissues cannot be studied properly in isolation, so the gene must be inactive in a certain tissue while remaining active in others. With this technology, scientists are able to knockout genes at a specific stage in development and study how the knockout of a gene in one tissue affects the same gene in other tissues.
The most commonly used technique is the Cre-lox recombination system. The Cre recombinase enzyme specifically recognizes two lox (loci of recombination) sites within DNA and causes recombination between them. This recombination will cause a deletion or inversion of the genes between the two lox sites, depending on their orientation. An entire gene can be removed or inactivated. This whole system is inducible so a chemical can be added to knock genes out at a specific time. Two of the most commonly used chemicals are tetracycline, which activates transcription of the Cre recombinase gene and tamoxifen, which activates transport of the Cre recombinase protein to the nucleus. Only a few cell types express Cre recombinase and no mammalian cells express it so there is no risk of accidental activation of lox sites when using conditional gene knockout in mammals.
A mouse containing the Cre gene and a mouse containing the loxP sequences are bred to generate a conditional knockout for a particular gene of interest. The mice do not naturally express Cre recombinase or lox sites but they have been engineered to express these gene products to create the desirable offspring. You have to obtain a mouse in which an important exon is floxed (it means that it has a Lox-P upstream and downstream) and a mouse which has a Cre sequence whose expression is regulated by a cell type specific promoter or an inducible promoter. Then you cross these mice and you obtain a mouse in which the cells non expressing Cre will have the gene with a normal function, while the cells expressing Cre will have a gene function disrupted in the portion between Lox-P.

(Francesca Luca)