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Stability of β-gal expression by noncytopathic replicons. During the course of cell passage (1:10, approximately every 48 h), a portion of cells were seeded in separate wells and stained with X-gal as described in Materials and Methods. Stability of β-gal expressing cloned cell lines was assessed by using clones selected for high level expression. The percentage of β-gal-negative cells was calculated by microscopic examination of several random fields.
