Effects of intronic mutations on mRNA splicing. To evaluate the products of genes with a single splice mutation in intron 4 (top) or intron 10 (bottom), we performed RT-PCR on mRNA using primers corresponding to these exons. RT-PCR for intron 4 yielded a 185-bp band from the normally spliced transcript and a 510-bp band from the unspliced transcript. RT-PCR for intron 10 yielded a 115-bp band from the spliced transcript and a 226-bp band from the unspliced transcript.