Northern blot hybridization of total RNA from 4 different tissues of the human eye. No band, indicating hCx59 expression, was detected even after three weeks of exposure, whereas rehybridization to a probe coding for the human glyceraldehydephosphate dehydrogenase [GAPDH]  led to a specific hybridization signal of about 1.4 kb. Hybridization signals for hCx62 were detected in retina and the optical nerve, whereas hybridization signals specific for hCx45 were seen also in retina and the iris diaphragm after two weeks of exposure. Interestingly, a faint signal at 6.5 kb might indicate expression of hCx62 also in the human retina. S; RNA-Ladder (Gibco-BRL). RT-PCR analyses of the expression of hCx59 and hCx62 in human retina cDNA after DNAseI digestion. The spliced 243 bp amplicon but not the 330 bp amplicon of the beta-actin gene demonstrates the absence of genomic DNA from the human retina cDNA pool. Primer combinations USP1-DSP1 and the intron-spanning USP1-DSP3 yielded fragments of the expected size. Primer combination USP1-DSP2 failed to give any signal with cDNA from human retina but instead with human genomic DNA. Different primer combinations for hCx59 yielded amplicons of the expected size (USP2-DSP1; 550 bp and USP1-DSP2; 1.5 kb) when probed with cDNA from human retina. An amplicon of about 1.1 kb was found after applying human genomic DNA. No signals after beta-actin RT-PCR and the empty water control indicate the absence of genomic DNA from the probes.