Quantitative PCR analyses of single cells. Cells from matched (EPO versus EST) cultures were sorted and analyzed on the days shown (x-axis). The copy numbers of mRNA amplified from cells cultured in EPO (□) versus EST (▪) are shown. (A) Average gamma-globin mRNA copy number per cell. (B) Average beta-globin mRNA copy numbers per cell. (C) Average total (gamma + beta) globin mRNA copy numbers per cell. (D) The ratio of gamma to total globin mRNA copies expressed as a percentage. (E) Frequency of cells having gamma-globin transcripts expressed as a percentage (γ Freq). (F) Frequency of cells having beta-globin transcripts expressed as a percentage (β Freq). The bars demonstrate the mean ± standard deviation (vertical line) of 24 single cells analyzed for each culture condition on each culture day. ^*Statistical significance (P ≤ .01) between EPO versus EST cultures.