The supernatant of transduced CD34⁺ cells taken 48 hours after exposure to 20 pfu of vector/cell does not affect β-GAL expression by nontransduced CD34⁺ cells (NT + supernatant) measured 48 hours after exposure. β-GAL expression by transduced and nontransduced hematopoietic progenitors 48 hours after transduction at the same MOI is shown for comparison.