Expression of the human globin genes in MEL(Ppo-155 β-YAC) clones. Total RNA was isolated from MEL cells at the days shown above the autoradiogram and was subjected to RNase protection assay as described in Materials and Methods. In (A), the RNA was hybridized with the human β-globin probe. In (B), the RNA was hybridized with both γ- and ɛ-globin probes. The location of the protected fragments is shown next to the autoradiogram. Notice the striking variation between human globin mRNA levels between clones.