CD34⁺ progenitor cells are not able to productively sustain X4 HIV-1 infection. The UCB-, BM-, and MPB-derived CD34⁺ cells were incubated for 1 hour with the HIV-1_LAI/IIIB strain of HIV-1 virus at the m.o.i. of 1. BM cells were infected with BON primary isolate diluted 1/30 (exp. 9). In exp. 5, MPB CD34⁺ cells were sorted for the presence of both CD4 and CXCR4 and cultured for 24 hours before infection, as indicated in Materials and Methods. After infection, cells were washed twice and plated in a 48-well plate. Culture supernatants were harvested every 3 days and the particles production was monitored by RT activity. A kinetic of HIV-1 IIIB replication in PBMC is shown as control of the HIV-1_LAI/IIIB and BON infectivity.