Comparative RT-PCR of salmon gill cartilage RNA and liver RNA. Duplicate southern blots were prepared that contained the RT-PCR products from 100-ng gill cartilage RNA (lanes 3 and 7) or liver RNA (lanes 4 and 8). The RNAs were reverse-transcribed into cDNA using random hexamers as primer, subjected to 30 cycles PCR with primer set IR-8/IR-3, electrophoresed on a 1.4% agarose gel, and blotted onto Hybond-N membrane. The upper blot was hybridized with radiolabeled probe IR1–4 (sequence = 5′-CCAAGGACATCGTPAAGGG); the lower blot was probed with IGFR5–6 (sequence = 5′-GPTGACAGTTGAACTCCTTC). As positive controls, lanes 1 and 5 contained 50 ng each pSIR-1/pSIR-2/pSIR-3/pSIR-4 plasmid DNA digested with EcoRI + HindIII; lanes 2 and 4 contained 100 ng each pSIR-5/pSIR-6 plasmid DNA digested with EcoRI + PstI. The blots were exposed to x-ray film for 1 h and were scanned on a Molecular Dynamics PhosphoImager. ND, not detected.