Serum growth factor activity is elevated in EpoR-HM mice. Sterile plasma samples were prepared from 8-week-old EpoR-HM and wt-EpoR mice and were used to challenge G1E2 cells in a ³HdT incorporation assay format. Data shown are for samples from individual EpoR-HM and wt-EpoR mice, and outcomes are representative of 2 independent experiments.