SAMHD1 and HIV-1 infection: HIV-1 infection did not efficiently promote PARP cleavage in both PMA-differentiated U937 and PMA-differentiated THP-1 cells in the presence or absence of SAMHD1 expression compared with that observed in undifferentiated U937 and THP-1 cells (Fig. 4C and D vs Fig. 1 to 4A and B).