Human globin gene expression of β-YACs of LxMEL hybrids. The β-YACs were first transferred into L-cells by lipofection and subsequently into MEL cells by cell fusion. Globin mRNA expression was analyzed in L(Ppo-155 β-YAC)xMEL hybrids at various time points after hybrid formation. Hybrid 7 shows extinction of expression of the human and murine globin genes. The visual differences in expression levels in the autoradiogram were diminished when the human mRNA levels were corrected for the copy number of the integrated β-YACs and the level of expression of the murine  globin gene. Quantitative data are shown in Fig 6.