To determine the CD4-specific biodistribution kinetics in vivo, we analyzed 64Cu-CD4-Nb1 uptake in orthotopic S2WTP3 polyoma virus middle T antigen (PyMT) breast cancer–bearing hCD4 knock-in (hCD4-KI) and WT C57BL/6J mice (fig. Here, CD4 is linked to breast carcinoma.