STAT5A and neoplasm: Previous studies have shown that tumor acidity—a general microenvironmental feature of solid tumors—is a main cause of the accumulation of anergic tumor-infiltrating T cells with impaired cell viability, cytokine secretion and cytolytic activity by suppressing the activation of the signal transducer and activator of transcription 5A–extracellular regulated protein kinase (STAT5–ERK) axis [23].