Stat 5a protein is a transcription factor activated by hormone and cytokines. Two highly related, but distinct Stat 5 genes (Stat 5a and Stat 5b) were identified in mouse. The amino acid sequences of Stat 5a and Stat 5b show 96% sequence similarity, and both proteins are co expressed in most tissues of both virgin and lactating mice. However, differential accumulation of Stat 5a and Stat 5b mRNA has been reported for both muscle and mammary tissue. Stat 5a is critically involved in a variety of physiological functions, including reproduction, lactation, immune function and somatic growth.