An RNA version of the Southern blot is called the Northern Blot . You therefore have to produce clones of all of these, and then identify the one you want. Typically, the vectors carry antibiotic resistance genes and also marker genes, the latter being inactivated when the DNA to be cloned is ligated into the vectors. To identify your clone of interest, you must then probe the library in much the same way that you would a Southern Blot. It is also possible to construct a library that expresses the gene you have cloned in - this can be probed with an antibody.
Transgenic animals are animals in which a specific gene has been inserted, mutated or deleted.
Production of transgenic mice rests largely on the development of a cell line, ES cells, that can if injected into an early embryo contribute to the developing animal. If one manipulates the genome of ES cells and incorporates them into an embryo, the resulting animal will be a chimaera of normal and manipulated cells. If the manipulated cells colonise anyof the testis, then offspring of the chimaera will include those whose paternal contribution came from the manipulated rather than the normal cells. Interbreeding of this generation can then produce homozygotes for the manipulation. Also in the vector , outside the region of complementarity to the mouse genome, is a gene called thymidine kinase.