As the genetic heterogeneity of tumors can lead to the selection of phenotypes resistant to single-agent therapies, recently, immunotherapeutic agents such as immune checkpoint inhibitors, histone deacetylase (HDAC) inhibitors, MAPK pathway inhibitors, and TGFβ inhibitors have been used in combination with oHSVs, inducing antitumor immune responses and demonstrating a higher efficacy in targeting cancer cells [6,25,26,27]. This evidence concerns the gene HDAC9 and cancer.