MTT assay

K562 cells at log phase were digested, counted, and inoculated into a 96-well plate (3000 cells per well). Cells were randomly divided into 3 groups: control group, low dosage (20 μM) As₄S₄ group, and high dosage (40 μM) As₄S₄ group (N=5 each) for 48-h drug treatment. We added 5 g/L MTT solution into each group (20 μl each) for 4-h incubation. Supernatant was then removed, followed by the addition of 150 μl DMSO for 10-min vortexing until complete resolving of violet crystals. A microplate reader was used to detect absorbance (A) value at 570 nm wavelength of each well for calculating cell proliferation rate in triplicate.