STAT5A and breast cancer: After inoculating tumors in immunocompromised mice, the levels of STAT5a and STAT5b of T and B lymphocytes isolated from mice were significantly reduced, indicating that the levels of STAT5 are related to tumor progression.120,123 Besides, STAT5 is involved in breast tumorigenesis, and mainly participates in the early development of breast cancer.106 (4) Regulation of cell growth, differentiation, and apoptosis.