CDKN1A and neuroblastoma: Furthermore, combined treatments of neuroblastoma cells with VPA and the COX-2 inhibitor celecoxib, or either sodium butyrate/vorinostat with the differentiation inducing agent, retinoic acid, also enhanced the induction of p21 (WAF1/CIP1) in neuroblastoma cells when compared to the single agent treatment arms alone (De los Santos et al., 2007; Chen et al., 2011).