The Drosophila melanogaster Trithorax-like (Trl) gene is classed with the trx-G genes and codes for several isoforms of the GAGA transcription factor (GAF) which regulates expression of homeotic and numerous other genes. GAF acts as a transcriptional antirepressor, i.e., its interaction with nucleosomal DNA results in the open chromatin conformation in promoter gene regions. The regions thereby become accessible to other transcription factors. As mutations of the Trl gene enhance position effect variegation and disturb chromosome segregation in mitosis and meiosis, GAF is thought to play another, more significant role in determining the chromatin structure. To study the molecular basis of its pleiotropic effect, the Trl gene was subjected to a structural analysis. The genomic Trl gene was sequenced, the sizes of its exons and introns was established, and a complex structure of the 5' and 3' gene regions was demonstrated. The Trl13C, Trl62, DfTrlR67, and DfTrlR85 mutations were exactly mapped. In addition, four insertions of the P element were identified as Trl alleles (Trll(3)s2325, TrlEP(3)3184, TrlEP(3)3191, and TrlEP(3)3609). The viability at various developmental stages was studied in homozygotes for the Trl mutations and in interallelic compounds. The following lethality stages were established: hatching, (Trl13C, DfTrlR85, TrlEP(3)3609), larval molts (Trll(3)s2325), pupation, metamorphosis (DfTrlR67, Trl62), and eclosion (several compounds).