Position-dependent expression of human globin genes after direct transfer of the YAC into MEL cells. Total human globin mRNA levels (ie, ɛ + γ + β) of the MEL (β-YAC) clones are expressed as the percentage of the endogenous mouse -gene corrected for copy number. Each column represents results obtained by a single clone identified with the number below the column. Clone 1 had 0% and clone 7 less than 2% of mouse  mRNA and are shown as flat lines. Results from 6 culture days are shown. Notice the striking variation in human globin gene expression between clones. Such findings indicate that the expression of the globin genes is strongly influenced by the position of integration of the β-YAC into the mouse genome.