Carries out a dual function: signal transduction and activation of transcription. Mediates cellular responses to the cytokine KITLG/SCF and other growth factors. Mediates cellular responses to ERBB4. May mediate cellular responses to activated FGFR1, FGFR2, FGFR3 and FGFR4. Binds to the GAS element and activates PRL- induced transcription. Regulates the expression of milk proteins during lactation. . This is the function of STAT5A (signal transducer and activator of transcription 5A, Ensembl gene identifier ENSG00000126561).