Relative transduction efficiency of human, pigtailed macaque, rhesus, and baboon CD34⁺ cells with HIV-based vectors. Cryopreserved CD34⁺ cells were thawed and transduced with a lentiviral EGFP vector and the percentage of EGFP-expressing cells was determined 5 days after vector exposure. The mean and standard error from 3 experiments is shown.