RT-PCR analysis of yolk sac colonies cultured for 7 days in methylcellulose media containing IL-3, IL-6, stem cell factor, and GM-CSF (top) or IL3-, IL-6, stem cell factor, and erythropoietin (bottom). B.M., control PCR reaction performed on adult murine bone marrow cDNA. The control reaction for ε globin was performed on E10.5 embryo cDNA.