Stage-specific change in expression of alternative first exons. (A) Splicing index shows normalized changes in probe set expression along the entire SLC12A6 transcript for day 10 (blue curve) and day 14 erythroblasts (red curve), relative to expression at day 7 (black line). Results show a stage-dependent increase in expression of probe sets representing the 1A region (boxed) and a decrease in expression of probe sets for exon 1B. (B) RT-PCR validation of exon array predictions for SLC12A6 and 2 additional genes. Shown are gels of PCR products validating alternative splicing switches in first exon expression (left) and diagrams of the relevant pre-mRNA regions (right). Black arrow/black lines represent predominant pattern in day 7 erythroblasts; red arrow/red lines, predominant pattern in day 14 erythroblasts. Alternative first exons are indicated by 1A, 1B, 1C, and a shared constitutive exon indicated as exon 2. Common names of the alternatively spliced transcripts are as follows: SLC12A6 indicates KCl cotransporter 3 (KCC3); TNPO2, transportin 2 (a nuclear import protein); GCNT2, glucosaminyl (N-acetyl) transferase 2 (generates the branched chain carbohydrate structure that constitutes the I antigen).