STAT5 alpha and beta are recruited to the receptor complex and phosphorylated. JAK3 is believed to be responsible for the tyrosine phosphorylation of STAT5 in response to IL-2; it is not clear whether JAK1 is also involved (Lin & Leonard, 2000). Tyr-694 of STAT5a and Tyr-699 of STAT5b are required for IL-2 induced STAT5 activation (Lin et al. 1996). STAT5a and STAT5b are also known to be serine phosphorylated in lymphocytes activated by IL-2 but the funtion of this is unclear (Xue et al. 2002).