STAT5 is a protein that plays a crucial role in cell signaling, particularly in response to various hormones and growth factors. It is part of the STAT family of proteins, which help transmit signals from the cell surface to the nucleus, influencing gene expression and cellular functions. There are two main forms of STAT5: STAT5A and STAT5B, each encoded by different genes. These proteins are important for processes such as immune response, cell growth, and differentiation. Abnormal activity of STAT5 has been linked to various diseases, including certain types of cancer.