Coupling between alternative 5′ exons and alternative splicing at exon 2.

(Top) 1A transcripts were amplified from the indicated human tissues using primers in exons 1A and 2. Control shows migration of PCR products that include exon 2′ (346 nt; proximal splicing) or exclude exon 2′ (329 nt; distal splicing). (Bottom) 1B transcripts were amplified from the indicated tissues using primers in exons 1B and 2. Control shows migration of PCR products that include exon 2′ (295 nt; proximal splicing) or exclude exon 2′ (278 nt; distal splicing).