pphmtoolong, a guess: could be that Bavi loses some specificity for docking site when one arm is tied up with a payload. I cannot imagine that issue, if it is one, to be a long-term problem in subsequent constructs. There are some real pluses to "keeping it simple" with regard to safety studies. At this point, given Bavi safety, efficacy appears to be a virtual no-brainer with a payload. Then we don't need to try to beat a dead immune system into fighting a large solid cancer.