Analysis of RT-PCR products from lymphocyte mRNAs. Amplified fragments encompassing exon 5, exon 6, and the coding part of exon 7 were obtained from lymphocyte cDNAs by RT-PCR using primers HUCO-2-Bio-A and HUCO-10S (9) and analyzed on 2% agarose gel. Two amplified products were obtained from the heterozygous harderoporphyric patient (P), the 468-bp fragment containing the K404E missense mutation and the 363-bp fragment resulting from the mRNA with complete deletion of exon 6. Amplification of control mRNA (N) showed only the normal 468-bp fragment. M, molecular size markers.