To determine the downregulation of PD-L1 expression under SB-3CT treatment through the MMP2/9 inhibition, we knocked down (KD) MMP2 or MMP9 with two shRNAs in SK-MEL-28 (Fig. 4h–m) and A375 (Additional file 1: Fig. S4E-J) melanoma cell lines, respectively. The gene discussed is MMP9; the disease is melanoma.