TPO activity in the plasma from mice transplanted with MPZenTPO virus-infected bone marrow cells. Each symbol represents the data from a different recipient. Different symbols (▪, •, ♦, ▴, ▾) correspond to different infected groups. Identical values are superimposed. No TPO activity was detected in the plasma from MPZenNeo recipient mice when studied later than 3 weeks posttransplantation.