ABCB1 and breast carcinoma: Valproic acid (VPA), a histone deacetylase inhibitor, has been shown to partially reverse multidrug resistance (MDR) in breast cancer by downregulating the expression of efflux transporters, such as P-glycoprotein (P-gp, encoded by the MDR1 gene), and modulating key survival pathways, including PI3K/AKT signaling.