Enables DNA-binding transcription activator activity, RNA polymerase II-specific; RNA polymerase II cis-regulatory region sequence-specific DNA binding activity; and phosphate ion binding activity. Involved in growth hormone receptor signaling pathway via JAK-STAT and regulation of growth. Acts upstream of or within several processes, including animal organ development; positive regulation of leukocyte proliferation; and regulation of hemopoiesis. Located in cytoplasm and nucleus. Is expressed in several structures, including alimentary system; central nervous system; genitourinary system; hemolymphoid system; and liver and biliary system. Orthologous to human STAT5A (signal transducer and activator of transcription 5A). [provided by Alliance of Genome Resources, Apr 2022]