Developmental regulation of the globin genes in the MEL(β-YAC) clones. Human ɛ-, γ-, and β-globin gene expression levels are plotted as a percentage of total human mRNA output from the β-locus. Results are shown from three MEL(β-YAC) clones that express more than one mRNA species. (○) β-Globin mRNA; (⧫) γ-globin mRNA; (•) ɛ-globin mRNA. Notice the highly inconsistent pattern. In clone 12 there is no change in γ or β gene expression. Clone 17 shows a typical ɛ → γ → β switch. Clone 20 displays a delayed γ gene expression.