Transcription factor, master regulator of tracheal cell fates in the embryo, necessary for the development of the salivary gland duct, Malpighian tubules and the posterior spiracles (PubMed:8557189, PubMed:8557198, PubMed:9374395, PubMed:10502111). It may induce a general fate of branched tubular structures of epithelial origin (PubMed:8557189, PubMed:8557198, PubMed:10502111). Functions with tgo to regulate expression of btl (PubMed:9374395). Efficient DNA binding requires dimerization with another bHLH protein. Heterodimer with tgo. Trachea, salivary gland ducts, posterior spiracles (Filzkoeper primordia) and a subset of cells in the CNS. During embryogenesis, first detected in the tracheal placodes at stage 8, and expression continues throughout embryonic and larval development. In the developing salivary gland, expression is observed in the entire gland at stage 9 and by stage 12, expression is confined to the salivary ducts. Ser-673 phosphorylation by PKB/Akt1 is required for nuclear targeting and transcriptional activity. Tube-forming cells of the salivary gland, trachea, and filzkorper fail to invaginate to form tubes and remain on the embryo surface (PubMed:8557189). In embryo Malpighian tubules, cell rearrangement is incomplete and the resulting tubules are wider than normal (PubMed:10502111). Readthrough of the terminator UGA may occur between the codons for 958-Ser and 960-Val. Truncated N-terminus. Truncated N-terminus.