Gene targeting (also, replacement strategy based on homologous recombination) is a genetic technique that uses homologous recombination to modify an endogenous gene. The method can be used to delete a gene, remove exons, add a gene and modify individual base pairs (introduce point mutations). Gene targeting can be permanent or conditional. Conditions can be a specific time during development / life of the organism or limitation to a specific tissue, for example. Gene targeting requires the creation of a specific vector for each gene of interest. However, it can be used for any gene, regardless of transcriptional activity or gene size. Gene targeting methods are established for several model organisms and may vary depending on the species used. To target genes in mice, the DNA is inserted into mouse embryonic stem cells in culture. Cells with the insertion can contribute to a mouse's tissue via embryo injection. Finally, chimeric mice where the modified cells make up the reproductive organs are bred. After this step the entire body of the mouse is based on the selected embryonic stem cell. To target genes in moss, the DNA is incubated together with freshly isolated protoplasts and with polyethylene glycol. As mosses are haploid organisms, moss filaments (protonema) can be directly screened for the target, either by treatment with antibiotics or with PCR.