Transduction of Mpl-R does not modify the progenitor number in reconstituted animals. Mean number of clonogenic progenitors per leg or spleen, grown in methylcellulose in the presence of IL-3 + EPO or in agar in the presence of TPO, one (6 Neo- and 6 Mpl-R-reconstituted mice were studied) and 7 months after the graft (n = 6 for each group) of hematopoietic cells cocultured either on GpE86-Neo (NEO) or on GpE86-Mpl-R (Mpl-R) cells.