**3.3.3 Fluorescence In situ hybridization**

Fluorescence in situ hybridization (FISH) is most commonly used to identify the number of integration sites following random integration of a transgene. FISH has been successfully used to detect the number of bacterial artificial chromosome (BAC) sequences integrated and chromosomal location(s) in mouse ES cells (Yang and Seed, 2003). Although the chromosomal location can be shown directly by FISH, Gómez-Rodríguez et al. (2008) found that screening of BAC-based constructs by FISH can be prone to false positives because of small pieces of integrated DNA that are below the limits of detection by fluorescent hybridization.

## **3.4 SCNT using genetically modified somatic cells**

Once the cell line containing the appropriate genetic modification has been identified, those cells can then be used for SCNT as described in Figure 1. SCNT is a process in which the nucleus of a somatic cell is transferred into the cytoplasm of an enucleated oocyte. The hereby reconstructed SCNT embryos can be activated to initiate development and then are transferred into a synchronized surrogate mother immediately or after short-term *in vitro* culture. In addition to the technical factors involved in the SCNT process, the status of the donor cells and the quality of unfertilized oocytes are considered to largely affect the overall efficiency of the SCNT. This point will be discussed more below.
