Gene transfer efficiency with a 1-day infection protocol equals that of a 3-day protocol. The percent gene transfer represents the proportion of methyl cellulose colonies formed by human cells harvested from the BM of reconstituted recipients that were PCR-positive for the MDR gene from the retroviral construct. The days in culture indicate the total number of days human cells were cultured before transplantation into irradiated NOD/SCID recipients. The data represent three combined experiments in which the analysis of day 1 was performed in 2 separate experiments. A MOI of 130 or 260 did not show differences in gene transfer efficiency.