To determine whether P300/CBP activity had an effect on the expression of S100 genes in HER2-overexpressing breast cancer cells, we treated SKBR3/100–8 cells with A-485, a selective small-molecule inhibitor of P300/CBP acetylase activity, and examined expression of S100 family genes by RT-qPCR. The gene discussed is S100A1; the disease is breast carcinoma.