the importance of negative regulatory elements in β4 gene expression and may be a consequence of the absence of a cell-type-specific repressor in the transgene used in this study. We previously demonstrated in vitro that this repressor, referred to as “α3 intron 5”, exerts its transcriptional effects in non-β4-expressing cells but is relatively inactive in β4-expressing cells (Fuentes Medel and Gardner, 2007).
