In Sydney, Australia, the number of pigmented moles per person increases to age 15 in males and 20‐29 in females, the most exposed parts of the body reaching peak values soonest. A decline then begins so that moles are almost absent in persons 80 years old. On each part of the body, depigmented spots and nevi with definite or faint halos are more common soon after the peak values have been reached. The delayed development of moles on sunlight‐protected parts of the body may be related to the fact that in European pop ulations the frequency of malignant melanoma increases as the Equator is approached but many of the additional melanomas are found in anatomical sites minimally exposed to sunlight.

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We have developed a highly sensitive non-radioactive in situ hybridization technique that enables us to study the production of mRNAs in tissues. As part of the validation procedure of our methods, we examined various methods of detecting poly-A RNA tails of mRNA. We have used three types of biotin-labelled probes complementary to poly-A sequences: a 25-mer poly-dT oligonucleotide, a polymer of dT, and a heteropolymer of dT:rA. All the probes had the same specificity of reactivity but the heteropolymer of dT:rA gave the strongest signals as visualized histochemically by the use of alkaline phosphatase as the detection enzyme. All the probes tested for poly-A detection showed reactivity. The poly-dT oligonucleotide showed a strength of signal comparable to published results. The biotinylated polymer of dT gave a stronger signal than that of the oligonucleotide, and the heteropolymer was the strongest of all. The strong signal seen with the heteropolymer probe is due to probe complexing during hybridization, in which additional binding between sense and antisense strands of the probe (i.e. poly-rA and poly-dT) amplifies the number of biotin molecules at the hybridization site; this strategy has been exploited by us as a means of visualizing low copy numbers of specific mRNAs.