Expression of Mpo in single cell clones of 32D transfectants. Single cell clones of the 32D transfectants (obtained by limiting dilution) were grown in either 10% FCS (open bars) or 100% serum-free media (dotted bars) in the presence of 2 ng/mL IL-3 (32D/neo) or 2 U/mL Epo (32D/EpoR WT and 32D/EpoR FE) for 3 days and then cytochemical stains for Mpo were performed. A total of 200 cells were counted for each individual clone tested. The number of clones tested was 2 for neo, 5 for EpoR WT, and 5 for EpoR FE. The bars indicate standard error.