The gene transfer at homologous sites in the host genome is popularly described as targeted gene transfer. Initially, such targeted gene transfer by homologous recombination for replacement of a resident wild or mutant gene was possible only in bacteria and yeast.
Later, for the first time in 1985, such targeted gene transfer was successfully achieved in human cells by Oliver Smithies at Madison (USA) transferred a human β globin gene into the β globin gene of recipient cells by recombination. The targeted gene transfer was possible due, to the presence of homologous DNA sequences at the targeted site and in the vector carrying the foreign gene.
Marker genes are also used for selecting cells transfected with transferred gene at the targeted site. Such a selection has been possible by several methods:
(i) by assaying the activity of an enzyme, as in hypoxanthine, phosphoribosyl transferase (HPRT) enzyme;
(ii) by using a marker gene for antibiotic resistance, that would enable cells to grow in a medium containing the antibiotic that would otherwise kill the cells;
(iii) by polymerase chain reaction (PCR), if sequence of the gene is available for making PCR primers.
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