Stat5a is essential for adult mammary gland development and lactogenesis. Researchers disrupted the Stat5a gene in mice to study its role. Stat5a-deficient mice developed normally but failed to lactate after birth due to a lack of terminal differentiation in mammary alveoli. While Stat5b is similar to Stat5a and has a similar expression pattern, it could not compensate for the absence of Stat5a. These findings show that Stat5a is the main and necessary mediator of mammopoietic and lactogenic signaling. Prolactin (PRL) induces mammary development and lactogenesis by activating STAT proteins, including Stat5a and Stat5b. The activity of these proteins is crucial for the expression of milk protein genes. In Stat5a-deficient mice, the expression of milk protein genes like β-casein and WAP was reduced, indicating that Stat5a is critical for their expression. Despite the presence of Stat5b, it could not fully compensate for the loss of Stat5a. The study also showed that Stat5a is necessary for the development of lobuloalveolar structures in the mammary gland. The results highlight the unique and essential role of Stat5a in mammary development and lactation.Stat5a is essential for adult mammary gland development and lactogenesis. Researchers disrupted the Stat5a gene in mice to study its role. Stat5a-deficient mice developed normally but failed to lactate after birth due to a lack of terminal differentiation in mammary alveoli. While Stat5b is similar to Stat5a and has a similar expression pattern, it could not compensate for the absence of Stat5a. These findings show that Stat5a is the main and necessary mediator of mammopoietic and lactogenic signaling. Prolactin (PRL) induces mammary development and lactogenesis by activating STAT proteins, including Stat5a and Stat5b. The activity of these proteins is crucial for the expression of milk protein genes. In Stat5a-deficient mice, the expression of milk protein genes like β-casein and WAP was reduced, indicating that Stat5a is critical for their expression. Despite the presence of Stat5b, it could not fully compensate for the loss of Stat5a. The study also showed that Stat5a is necessary for the development of lobuloalveolar structures in the mammary gland. The results highlight the unique and essential role of Stat5a in mammary development and lactation.