Table 3. Comparison of HPRT targeting frequency in ES cells.

Author	Species	Strain	Homology	Expt.	Cell No.	G418R	6-TGR¶	Frequency†
Burdon	Rat	F344	4.7 kb	1	2.7×106	58	1	0.4×10−6
				2	2.7×106	53	1	0.4×10−6
		SD	4.7 kb	3	3.0×106	43	3	1.0×10−6
				4*	3.0×106	60	1	0.3×10−6
Capecchi [19]	Mouse		9.1 kb	N/A	7.8×107	30000	32	0.4×10−6
Smithies [18]	Mouse		1.3 kb	N/A	2.5×106	2000	4	1.6×10−6
Thomson [20]	Human		11.9 kb	N/A	1.5×107	350	7	0.5×10−6

^¶Selection with G418 first, followed by 6-TG selection.

^†Number of targeted cells per number of cells electroporated.

^*Electroporation with frt flanked Neo^R hprt targeting vector.