Not enough muscles (nem) mutants of Drosophila reveal defects in the development of embryonic muscles, a subset of pericardial cells, the CNS and derivatives of the PNS (Burchard, S., Paululat, A., Hinz, U. and Renkawitz-Pohl, R. (1995) The mutant not enough muscles (nem) reveals reduction of the Drosophila embryonic muscle pattern. J. Cell. Sci. 108, 1443-1454). The molecular analysis of the nem locus shows a complex genomic structure. One transcription unit was identified as inscuteable (insc). Within the first intron of insc we find another independent gene, skittles (sktl), which is not affected in nem mutants. insc transcripts are localised apically in neuroblasts and may prefigure the localisation of the protein. The skittles mRNA is ubiquitously distributed during early embryogenesis due to maternal contribution. Later, some enrichment of sktl is observed in the nervous system and the mesoderm. The muscle phenotype shows deletions as well as duplication of specific muscles which is reflected in a change of even-skipped (eve) and Krüppel (Kr) expressing cells. Our data suggest a role for insc in the specification process of a subset of muscle progenitors/founders. Furthermore, in insc mutants the eve expressing pericardial cells of the developing heart are significantly reduced in numbers.