Table 2.

Comparison of different methods for genetic manipulation in the rat

Method		Time	Cost	Efficiency (mutational/targeting)	Screening	Confounding factors	Advantages
Forward genetics (random)	ENU mutagenesis	<1 Year	Relatively low	Very high (one mutation in every 1–2 Mb in genome)	In adult animals	Substantial background mutations	Indefinite cryopreservation of rat sperms from G1 mutagenized animals
	Transposon mutagenesis	<1 Year (in SSC system)	Low	Low (1–10 hits per genome)	In rat SSC lines	Genomic ‘hot spots’ and ‘cold regions’ of insertional events	Rapid detection of insertion by reporter or gene-trapping cassettes
Reverse genetics (targeted)	ZFN-mediated gene-targeting	6–9 Months	High	Relatively high	In adult animals	Off-target effects	Embryonic stem cell-independent
	Embryonic stem cell-based gene targeting	1 Year	Low	Low (1–3%)	In rat embryonic stem cell lines	Germline competent embryonic stem cells of the chosen strain are necessary	Enables sophisticated genetic manipulations (knock-ins, conditional knockouts)

ENU, N-ethyl-N-nitrosourea; SSC, spermatogonial stem cell; ZFN, zinc-finger nuclease.