STAT proteins are important for responding to signals in the body. Scientists studied mice with genes for different STAT proteins turned off. They found that STAT1 is needed for responding to interferons, STAT4 and STAT6 are important for immune responses, STAT5a is crucial for mammary gland development, and STAT5b affects growth hormone effects in the liver. STAT3 is essential for cell growth, preventing cell death, and cell movement. These findings help us understand the specific roles of different STAT proteins in the body.