These compounds have initially shown promising therapeutic potential in human cancer studies (McLaughlin and La Thangue, 2004; Villar-Garea and Esteller, 2004; Ryan et al, 2005; Lin et al, 2006; Riester et al, 2007; Jones and Steinkuhler, 2008; Prince et al, 2009; Rocca et al, 2009; Takai and Narahara, 2010); yet, the relationship between their HDAC inhibitory activity and their ability to control cancer growth is not clear. Here, HDAC9 is linked to cancer.