The fusion itself, specifically the linker region. When the Provenge data came out, it was remarkable that neither half of the fusion elicited an immune response, but the linker did. I'm wondering if the linker will be similarly antigenic in this case.
I did too, as is my default stance. The charts clearly had patients followed out to enough days for confirmatory scans. And since they noted stable disease, it's clearly they are monitoring for efficacy. To be fair though, the numbers are small at this point and responses need not be immediate to eventually occur. However, those two caveats likely also speak to the possibility that this drug may not be the efficacy darling that the "don't eat me" tag line was implying.