- The Jak1 SH2 domain does not fulfill a classical SH2 function in Jak/STAT signaling but plays a structural role for receptor interaction and up-regulation of receptor surface expression. Radtke, S., Haan, S., Jörissen, A., Hermanns, H.M., Diefenbach, S., Smyczek, T., Schmitz-Vandeleur, H., Heinrich, P.C., Behrmann, I., Haan, C. J. Biol. Chem. (2005)