Stage-specific switch of protein 4.1R exon 16 splicing in highly purified erythroblast cultures. (A) Fluorescence-activated cell sorter analysis of day 7 erythroblasts from 3 different preparations indicates that purity is more than or equal to 97%, based on expression of erythroid markers for glycophorin A and CD71. Quantitative analysis demonstrated erythroblast purities as follows: prep 1, 97%; prep 2, 97%; prep 3, 99%. (B) RT-PCR scheme used to analyze 4.1R pre-mRNA splicing in early (day 7) and late erythroblasts (day 14), using primers in the nearest constitutive exons 13 and 17. Gel image shows primarily exclusion of exon 16 in early erythroblasts (bottom band), whereas substantial inclusion of exon 16 was observed in late erythroblasts (top band). Alternative exons 14 and 15 are not expressed in erythroid cells.