Tissue-specific expression of alternative 5′ exons.

A master RNA blot (Clontech) containing poly A⁺ RNA from many human tissues and cell lines was blotted with probes for alternative exon 1A, exon 1B, or exon 1C. The 1A and 1C transcripts were particularly abundant in tissues with a high content of muscle cells (heart, A4; skeletal muscle, B7; and stomach, B5) as well as in pancreas (B9) and liver (A9). In contrast, exon 1B transcripts were most abundant in tissues enriched for early erythroid progenitors (fetal liver, D11; bone marrow, G7) and the erythroleukemia cell line K562 (C10). Lower panel shows the tissue map for all RNA sources tested.