Expression of alternative 5′ exons in erythroid progenitor cells.

Shown are Northern blot analyses of total RNA from Friend virus anemia–induced erythroblasts following culture for the times indicated (in hours). (A) Full-length cDNA probe. (B) Exon 1A probe. Both probes detected a single 4.1R mRNA band that increased during the course of differentiation. No hybridization was detected with probes for exons 1B and 1C under identical conditions (not shown). (C) Ethidium-bromide staining of ribosomal RNA. Equal (0, 16, and 30 h) or slightly reduced (44 h) loads of RNA were applied for each time point, supporting the idea that a true increase in 4.1R RNA expression has occurred in these cells.