Mean number of progenitors present in individual spleen colonies generated by hematopoietic cells cocultured on GpE86-Mpl-R cells. Cells from each individual spleen colony were grown in methylcellulose in the presence of IL-3 + EPO (CFU-GM and BFU-E) and in agar in the presence of TPO (CFU-MK). Each individual colony was tested for retroviral integration by PCR technique and was determined as negative (Mpl-R⁻, n = 30) or positive (Mpl-R⁺, n = 16).