Table 2. Principal traits of SV40-derived vectors

Attribute	Typical rSV40 characteristics
Titer	Typically, 10¹¹ to 10¹² infectious particles/mL
Longevity of transgene expression	Once established, transgene expression persists indefinitely.
Gene delivery efficiency	In vitro: Usually >98%, without selection
	In vivo: In discrete organs (e.g., liver), >90% of cells can be transduced.
Maximum insert size	Up to 5 kb
Target cell tropisms	Almost all resting and dividing cells can be transduced.
Antigenicity imparted to target cells	None observed to date
Neutralizing antibody	None observed to date
“Gutless” vectors	Generated exactly as nongutless vectors. Yields are comparable to those of nongutless vectors.
Hardiness	Very robust; in lyophilized form, can be stored for long periods of time at 4°C and even at room temperature.
Promoters used	Constitutive pol II, pol III promoters; conditional pol III promoters
Transgenes expressed	Almost all mammalian cDNAs; cDNAs of mammalian viruses
Limitations	Nonvertebrate marker genes generally not expressed (e.g., GFP and β-galactosidase)
	Levels of protein expression are generally lower than seen with many other vector systems.