Constitutive STAT5 activation is oncogenic, promoting tumor initiation through overexpression of transforming growth factor-α (TGFα) and increased cellular proliferation [23, 24], although among women with estrogen receptor (ER) positive breast cancer, the presence of pSTAT5 was associated with better response to hormone therapy and improved survival [16]. The gene discussed is STAT5A; the disease is breast carcinoma.