In vitro and in vivo analysis of barrier activity. (A) Constructs on the left were cotransfected into K562 cells with a pRSVneo plasmid, and individual clones were isolated in G418. After transfer to nonselective medium, the clones were assayed for GFP expression. The table at the right shows the number of GFP-expressing (GFP+) and -silenced (GFP−) clones. Significant resistance to silencing is indicated by asterisks (*P < .05). (B) Constructs on the left were used to generate transgenic mice. The table on the right shows the number of lines expressing huβ-globin mRNA (isolated from red blood cells obtained by bleeding of anemic adult mice) and the number of lines with uniform expression of huβ-globin protein (isolated from red blood cells obtained by bleeding of adult mice) for each line with sufficient levels of huβ-globin protein for analysis.