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The majority of D. melanogaster salivary gland nuclei contains many nucleoli which vary in size and number. All nucleoli hybridize in situ with a cloned Drosophila DNA fragment containing 26S ribosomal gene. Autoradiographic analysis of preparations after pulse H3-uridine or H3-thymidine labelling of the salivary gland indicates an intensive transcription and replication of DNA within nucleoli. The nucleoli are bound to different sites of polytene chromosomes by chromatin fibers similar to strands of ectopic pairing and they are most often bound to regions which may be defined as intercalary heterochromatin.

An investigation of the properties of a number of genes of Drosophila, obtained by cloning recombinant DNAs, led to the detection of a new type of organization of genetic material. It was found that a number of actively working structural genes of Drosophila are represented by a large number of copies, scattered over its chromosomes. Their localization in the chromosomes is variable, although they are always detected in regions of intercalary heterochromatin. The latter evidently is a site of accumulation of various multiple genes.