JAK2 is believed to bind STAT1 and STAT3 directly in response to GH, as opposed to STAT5 which binds to phosphorylated tyrosine residues in the distal portion of the GHR cytoplasmic region (Smit et al. 1996). When associated with the prolactin receptor JAK2 is able to bind STAT1, STAT3 and STAT5 (DaSilva et al. 1996).
Smit et al. identified mouse GHR residues Y341 (human Y332) and Y346 (not conserved in human) as required for STAT1, 3, and maximal STAT5 activation.