Evidence for the potential role of Mbnl1 in regulating splicing during erythroid differentiation. (A) Pentamer motifs significantly enriched in the 4 flanking 250-nt intronic regions of regulated skipped exons. Rectangular boxes indicate the names of splicing factors with ≥1 known motif enriched at an FDR < 0.05. Bar graphs show the fold enrichment of the pentamer motifs. The asterisks on the gray exon trio on the left denote the location of the motifs. (B) Mbnl1 isoform expression levels during erythropoiesis. Mbnl1 transcript isoform expression level was measured by seminquantitative PCR, and Ψ values were quantified by relative intensities of the inclusion and exclusion bands. The lower bar graph shows qRT-PCR analysis of Mbnl1 inclusion and exclusion isoforms. **Significance (P < .01). (C) Western blot showing the expression the Mbnl1 inclusion and exclusion isoforms during terminal erythropoiesis; western blots of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and hemoglobin α are included as controls.