I have ET Jak -ve. I asked my haematologist about the significance of this, and she told me there was none, really. She said that all it did was showed doctors whether there was a causal reason for having ET. That s, your blood is reacting to other influences or it is just plain bad luck. The thing is, whether a person is negative or positive seems to be given a lot of importance here. This has left me a tad confused, which, to be honest, isn't difficult!