Members of five multigene families in Drosophila melanogaster have been isolated as cloned DNA segments (Dm segments). These families divide into two classes: tandem and dispersed. The genes in the first class are contained in repeating units that are tandemly arranged. Sequences homologous to the functional RNA (e.g., mRNA or rRNA) produced by three tandem families have been mapped, both within the Dm segments and within the genome: (1) the 18S and 28S rRNA genes, (2) the five histone genes, (3) one of the set of heat-shock genes. Members of dispersed families occur singly at many different chromosomal sites that are widely distributed over the genome. Sequences homologous to the mRNAs produced by two dispersed families have been similarly mapped. Both of these families code for mRNAs that are represented in the abundant class in several D. melanogaster cell types. Transcribed sequences in the DNA that flanks the mRNA regions in the Dm segments from both classes will be mapped, not only to locate promoter and terminator regions, but to define the boundary conditions for processing. Another purpose of this mapping is to determine whether members of a dispersed family are commonly controlled and hence simultaneously expressed (as is the case for members of at least some tandem families), or are independently controlled so that different members are expressed in different cell types during development. A more recent project centers on the genes that respond to the steroid hormone, ecdysone. Its objectives are the isolation of these genes, the molecular mapping of their coding and transcribed sequences, and the identification of the mechanism by which ecdysone controls their expression. BIBLIOGRAPHIC REFERENCES: Rubin, G.M., Finnegan, D.J. and Hogness, D.S. (1976). The Chromosomal Arrangement of Coding Sequences in a Family of Repeated Genes. Progress in Nucleic Acid Research and Molecular Biology 19, in press. Glover, D.M. and Hogness, D.S. (1976). A Novel Arrangement of the 18S and 28S Sequences in a Repeating Unit of Drosophila melanogaster rDNA. Cell, in press.