STAT5A and breast cancer: Three histologic features of mammary tumors have recently been related to the activation of specific pathways: myoepithelial differentiation has been associated with activation of the Wnt pathway [7], squamous metaplasia has been attributed to β-catenin stabilization [34-37], and alveolar differentiation and milk secretion are dependent on signal transducer and activator of transcription 5a [38].