STAT5A and head and neck squamous cell carcinoma: However, constitutively activated STAT5 (caSTAT5) has been observed in a wide variety of human cancers, from hematologic malignancies (leukemia, lymphoma, myeloma) to solid tumors such as breast cancer, squamous cell carcinoma of the head and neck (SCCHN) or melanoma [16]–[18].