Carries out a dual function: signal transduction and activation of transcription. Mediates cellular responses to the cytokine KITLG/SCF and other growth factors. Binds to the GAS element and activates PRL-induced transcription. Positively regulates hematopoietic/erythroid differentiation. Upon activation, forms a homodimer or a heterodimer with a related family member. Binds NR3C1 (PubMed:9528750). Interacts with NCOA1 (By similarity). Interacts with SOCS7 (By similarity). Interacts (via SH2 domain) with INSR (PubMed:9428692). Interacts with CPEB3; this inhibits STAT5B-mediated transcriptional activation (PubMed:20639532). Translocated into the nucleus in response to phosphorylation. In the virgin, found in most tissues. Particularly abundant in muscle tissue of virgin and lactating females, and of males. Detected both in virgin mouse and after mammary gland involution. The level of STAT5A increases constantly during pregnancy, but decreases during lactation. Tyrosine phosphorylated in response to signaling via activated KIT, resulting in translocation to the nucleus. Tyrosine phosphorylated in response to signaling via activated FLT3; wild-type FLT3 results in much weaker phosphorylation than constitutively activated mutant FLT3. Alternatively, can be phosphorylated by JAK2 (By similarity). Phosphorylation at Tyr-699 by PTK6 or HCK leads to an increase of its transcriptional activity (By similarity). Belongs to the transcription factor STAT family.