After we successfully validated radiolabeling and specific in vitro binding of 89Zr-hCD4-Mb and 89Zr-mCD4-Mb to CD4+ cells, we examined the in vivo PET uptake of 89Zr-hCD4-Mb by HPB-ALL leukemia xenografts (which constitutively express the hCD4 antigen) subcutaneously (s.c.) implanted into NSG mice. This evidence concerns the gene CD4 and leukemia.