Position-independent expression of globin genes of the YAC in L(β-YAC)xMEL hybrids. Total human mRNA levels of L(Ppo-155 β-YAC)xMEL hybrids shown in Fig 5 were corrected for the number of copies of the integrated human globin genes present in the hybrids and expressed as the percentage of one copy of the murine- gene. Results from culture days 54 and 72 postfusion are shown. Notice the small degree of variation in human globin expression between the three hybrids. Globin mRNA levels varied 1.5-fold on day 54 and 2.5-fold on day 72.