TABLE 1.

Recombinant monocistronic MLV-VL30m-lacZ retroviral titer^a

Vector	Retroviral titer (TU/ml)
pVL30m-SJ E1	0.3 × 105
pVL30m-SJ E2	1.3 × 105
pVL-SJ E3	<1 × 102
pMLV-LacZ E+	2 × 105

^aViral titers were determined by performing serial dilutions followed by LacZ histochemical staining. NIH 3T3 cells were seeded 24 h prior to infection with virus-containing medium harvested from stably transformed ecotropic GP+E-86 cells (56). The viral titers are reported as LacZ⁺ TU per milliliter of collected medium and correspond to the means of triplicate experiments. The standard deviations did not exceed 10% of the mean values.