HSP90AA1 and cancer: Taking into consideration the unconventional nature of mAb 4C5 in combination with the fact that during the humanization process the antibody affinity is frequently reduced, we next reconstituted the murine mAb 4C5 into a functional mouse-human chimeric version that, like the paternal antibody, binds to surface HSP90, is cell impermeable and inhibits cancer cell invasion.