Further mechanistic studies revealed that ZN444B inhibits Fos-related antigen-2 (FOSL2) expression in breast cancer cells by inducing the disassociation of HDAC1 and Sp1, inhibits the deacetylase activity of HDAC1 on Sp1 at K703, and abrogates the binding ability of Sp1 to the FOSL2 promoter. This evidence concerns the gene HDAC1 and breast cancer.