JAK2 and fatty liver disease: As mentioned above, the absence of E2 or GHR-JAK2-STAT5 signaling causes adiposity and hepatic steatosis which can be ameliorated by E2 (Heine et al., 2000; Simpson et al., 2005; Jones et al., 2007) or GH (LeRoith and Yakar, 2007) replacement, respectively.