Might play a role in signal transduction and activation of transcription. Plays an important role in the segmental pattern formation in the early embryo by activating specific stripes of pair rule gene expression in early development as part of the Janus kinase-STAT pathway (PubMed:8608595). Might play a role in male germline stem cell maintenance (PubMed:34644293). Forms a homodimer or a heterodimer with a related family member. Translocated into the nucleus in response to phosphorylation. Expressed throughout embryonic, larval, pupal, and adult stages, with some decrease in the late embryonic stages. The expression is uniform in unfertilized or just fertilized eggs, suggesting maternally deposited mRNA. At blastoderm stage, expression pattern shows stripes, that are reminiscent of many pair rule genes pattern. Tyrosine phosphorylated by hopscotch. Phosphorylation is required for DNA-binding activity and dimerization. Exibits aberrant expression of the pair rule gene even-skipped at the cellular blastoderm stage, leading to larval segmentation defects. Belongs to the transcription factor STAT family.