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===Cre recombinase===
[[Cre recombinase]] (Cre) is able to recombine specific sequences of DNA without the need for [[Enzyme#Cofactors|cofactors]]. The enzyme recognizes 34 base pair DNA sequences called ''lox''P ("locus of crossover in phage P1"). Depending on the orientation of target sites with respect to one another, Cre will integrate/excise or invert DNA sequences. Upon the excision (called "resolution" in case of a circular substrate) of a particular DNA region, normal gene expression is considerably compromised or terminated.<ref>{{cite journal |doi=10.1007/s10616-006-6550-0 |title=Recommended Method for Chromosome Exploitation: RMCE-based Cassette-exchange Systems in Animal Cell Biotechnology |year=2006 |last1=Oumard |first1=André |last2=Qiao |first2=Junhua |last3=Jostock |first3=Thomas |last4=Li |first4=Jiandong |last5=Bode |first5=Juergen |journal=Cytotechnology |volume=50 |pages=93–108 |pmid=19003073 |issue=1–3 |pmc=3476001}}</ref>
Due to the pronounced resolution activity of Cre, one of its initial applications was the excision of ''lox''P-flanked ("floxed") genes leading to cell-specific gene knockout of such a floxed gene after Cre becomes expressed in the tissue of interest. Current technologies incorporate methods, which allow for both the spatial and temporal control of Cre activity. A common method facilitating the spatial control of genetic alteration involves the selection of a tissue-specific [[promotor (biology)|promoter]] to drive Cre expression. Placement of Cre under control of such a promoter results in localized, tissue-specific expression. As an example, Leone et al. have placed the transcription unit under the control of the regulatory sequences of the [[myelin]] proteolipid protein (PLP) gene, leading to induced removal of targeted gene sequences in [[oligodendrocytes]] and [[Schwann cells]].<ref name = "leone">{{cite journal |doi=10.1016/S1044-7431(03)00029-0 |title=Tamoxifen-inducible glia-specific Cre mice for somatic mutagenesis in oligodendrocytes and Schwann cells |year=2003 |last1=Leone |first1=Dino P |last2=Genoud |first2=S.Téphane |last3=Atanasoski |first3=Suzana |last4=Grausenburger |first4=Reinhard |last5=Berger |first5=Philipp |last6=Metzger |first6=Daniel |last7=MacKlin |first7=Wendy B |last8=Chambon |first8=Pierre |last9=Suter |first9=Ueli |journal=Molecular and Cellular Neuroscience |volume=22 |issue=4 |pages=430–40 |pmid=12727441 }}</ref> The specific DNA fragment recognized by Cre remains intact in cells, which do not express the PLP gene; this in turn facilitates empirical observation of the localized effects of genome alterations in the myelin sheath that surround nerve fibers in the [[central nervous system]] (CNS) and the [[peripheral nervous system]] (PNS).<ref name=koenning>{{cite journal |doi=10.1523/JNEUROSCI.1069-12.2012 |title=Myelin Gene Regulatory Factor is Required for Maintenance of Myelin and Mature Oligodendrocyte Identity in the Adult CNS |year=2012 |last1=Koenning |first1=M. |last2=Jackson |first2=S. |last3=Hay |first3=C. M. |last4=Faux |first4=C. |last5=Kilpatrick |first5=T. J. |last6=Willingham |first6=M. |last7=Emery |first7=B. |journal=Journal of Neuroscience |volume=32 |issue=36 |pages=12528–42 |pmid=22956843|pmc=3752083 }}</ref> Selective Cre expression has been achieved in many other cell types and tissues as well.
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