(A) After transduction with 2 pfu/cell, β-GAL expression in hematopoietic progenitor cell cultures declines progressively with time of culture. (B) β-GAL expression declines in both CD34⁺ and CD34⁻ subsets of hematopoietic cells. To determine whether the loss of β-GAL expression was confined to the CD34⁺ population, we examined the proportion of cells that were marker positive in both the CD34⁺ and CD34⁻ marrow population, at different times after transduction with the HSV-DISC vector at 2 pfu/cell. Both CD34⁺ and CD34⁻ populations showed a decline in positivity, and the rate of decline was greater in the CD34⁺ cells.