However, a major shortcoming of THP-1 cells is their high expression levels of several HIV-1 infection restriction factors, such as APOBEC and SAMHD1 [47,48,49], although in the case of SAMHD1, the presence of restriction inactive (phosphorylated) or active (unphosphorylated) forms of the restriction factor are reported to vary according to the differentiation status of THP-1 [50]. Here, SAMHD1 is linked to HIV-1 infection.