SAP18, also known as Sin3A-associated protein, is component of the histone deacetylase complex that plays an important role in the regulation of eukaryotic gene expression. This protein directly interacts with SIN3 and enhances SIN3-mediated transcriptional repression when tethered to the promoter. It also has been shown to play a key role in gene-specific recruitment of the HDAC complex by a number of transcription factors including Gli, GAGA, and Bicoid.