 Nonetheless the terminal JH domains retain a high level of homology to tyrosine kinase domains.An interesting note is that only one of these carboxy-terminal JH domains retains full kinase function (JH1) while the other (JH2), previously thought to have no kinase functionality and accordingly termed a pseudokinase domain, has since been found to be catalytically active, albeit at only 10% that of the JH1 domain.Loss of Jak2 is lethal by embryonic day 12 in mice.