The maternal effect homeotic gene fs(l)h of Drosophila has been studied by sequence analysis of overlapping cDNAs corresponding to the major ovarian transcripts of 7.6 and 5.9 kb. The 5.9 kb mRNA sequence predicts a protein of approximately 110 kd, the 7.6 kb RNA a protein of 205 kd. The predicted proteins are very rich in glycine, alanine and serine, some of which occur as clusters. The proteins contain several potential asparagine-linked glycosylation sites and transmembrane domains. Antisera have been prepared against fusion proteins that contain portions of the predicted fs(1)h products; use of these sera supports the view that the fs(1)h products are membrane proteins. Staining patterns in progeny of fs(1)h mutant females show very early defects in the expression of products of the evenskipped, engrailed, and Ultrabithorax genes, suggesting a defect in initial segmental organization. The trithorax (trx) gene, a major regulatory developmental locus in Drosophila, has been cloned. Two large RNAs of 12 and 15 kb have been identified as major products of this locus in different developmental stages. A gene encoding a Drosophila RNP protein has been cloned and sequenced, and its chromosomal location determined. A related gene has been isolated by cross-hybridization; it appears to encode a distinct RNP protein, and is located on a different chromosome.