Activation of human globin gene expression of a silent MEL(β-YAC) clone after transfer into L-cells. The nearly silent MEL(β-YAC) clone 7 (see Fig 3) was fused with HygroR L-cells. A hybrid cell population was selected on the basis of its dual resistance to G418 (from the MEL βYAC cells) and hygromycin (from the L-cells). This hybrid retained the potential for globin gene expression upon induction of erythroid differentiation with hemin-HMBA. Total human globin mRNA expression of the parental MEL clone at the time of fusion was 1.6% of murine-. Notice that fusion with L-cells resulted in MEL(β-YAC)xL cell hybrids in which globin gene expression was activated with a fetal-like pattern. Total mRNA output from the locus was increased to 21% of the murine  gene. RNase protection was performed on day 30 postfusion.