A conditional gene knockout allows gene deletion in a tissue in a tissue specific manner. This is required in place of a gene knockout if the null mutation would lead to embryonic death, or a specific tissue or cell type is of specific interest. This is done by introducing short sequences called loxP sites around the gene. These sequences will be introduced into the germ-line via the same mechanism as a knockout. 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.
Genes not involved in early development have been effectively studied using knockout approaches that use gene deletion. However, it is typically not possible to knock off genes that are active during early development without the organism suffering a fatal outcome. One method around this is conditional knockout. Utilizing a site-specific recombinase called Cre, the original conditional knockout technique recombined short target sequences known as LoxP. Since then, other recombinases have been created and employed in conditional knockout experiments.