Signal transducer and transcription activator that mediates cellular responses to cytokines, hormones and growth factors (PubMed:29844444, PubMed:9188471). Binds to the GAS element and activates PRL-induced transcription. Positively regulates hematopoietic/erythroid differentiation. Involved in the oncostatin-M- mediated signaling pathway through type II OSM receptor complex (heterodimers composed of OSMR and IL6ST) (PubMed:9188471). {ECO:0000269|PubMed:20702587, ECO:0000269|PubMed:29844444, ECO:0000269|PubMed:8732682, ECO:0000269|PubMed:9188471}. This is the function of STAT5B (signal transducer and activator of transcription 5B, ENSG00000173757).