RT-PCR analysis of ecotropic retrovirus receptor (Ecotropic R; top panel) and amphotropic retrovirus receptor (Amphotropic R; middle panel) mRNA levels in mouse 9.5-day yolk sac cells. Receptor mRNA levels are shown for two or three separate samples of RNA isolated from unfractionated (Unfr) yolk sac and sorted Lin⁻ CD34⁺ and Lin⁻CD34⁻ yolk sac cells. The levels of mRNA encoding Ecotropic R and Amphotropic R can be estimated based on the level of β2-microglobulin mRNA (lower bands) in the same sample. The levels of Ecotropic R and Amphotropic R mRNA in NIH-3T3 cells are also seen (right column).