MECP2 and atypical Rett syndrome: In contrast, GFP fluorescence is absent in hemizygous Mecp2stop/y mice (Fig. 2B), in which Mecp2 is silenced by a stop cassette, and is observed in ~ 50% of nuclei in heterozygous Mecp2+/stop mice in which one Mecp2 allele is silenced to mimic the mosaic expression pattern seen in human female Rett syndrome [26,30] (Fig. 2C).