SAMHD1 and acute monocytic leukemia: Thus, we utilized a phenotypic assay in which THP-1 acute monocytic leukemia cells with SAMHD1 knockout or wild-type expression profile39 (Figure 5B) were incubated with a dose-response matrix of cytarabine and putative inhibitors for four days before cell viabilities were determined by resazurin reduction assay.