Entinostat-exemestane combination for use in a method of (a) treating breast cancer in a patient, wherein the method comprises continuing treatment with entinostat-exemestane combination therapy if the level of protein lysine acetylation determined after about 15 days of the entinostat-exemestane combination therapy is greater than the level of protein lysine acetylation prior to the entinostat-exemestane combination therapy; (b) treating breast cancer in a patient, wherein the method comprises (i) administering entinostat-exemestane combination therapy, and (ii) determining the change in protein lysine acetylation levels during the course of said therapy compared to pre-therapy protein lysine acetylation levels, wherein the level of protein lysine acetylation is determined in a tissue sample obtained about 15 days after initiating therapy; (c) treating breast cancer in a patient, wherein the method comprises (i) determining the level of protein lysine acetylation prior to administration of entinostat-exemestane combination therapy, (ii) administering entinostat-exemestane combination therapy, and (iii) determining the level of protein lysine acetylation during the course of said therapy, wherein the level of protein lysine acetylation is determined in a tissue sample obtained about 15 days after initiating therapy; or (d) treating breast cancer which displays resistance to prior aromatase inhibitor therapy, the method comprising administering to a patient a combination comprising entinostat and exemestane, wherein the patient did not demonstrate a complete response, a partial response or stable disease for greater than six months during prior treatment with an aromatase inhibitor, and wherein the method further comprises determining the level of protein lysine acetylation in a tissue sample obtained about 15 days after administration of the combination. (a) treating breast cancer in a patient, wherein the method comprises continuing treatment with entinostat-exemestane combination therapy if the level of protein lysine acetylation determined after about 15 days of the entinostat-exemestane combination therapy is greater than the level of protein lysine acetylation prior to the entinostat-exemestane combination therapy; (b) treating breast cancer in a patient, wherein the method comprises (i) administering entinostat-exemestane combination therapy, and (ii) determining the change in protein lysine acetylation levels during the course of said therapy compared to pre-therapy protein lysine acetylation levels, wherein the level of protein lysine acetylation is determined in a tissue sample obtained about 15 days after initiating therapy; (c) treating breast cancer in a patient, wherein the method comprises (i) determining the level of protein lysine acetylation prior to administration of entinostat-exemestane combination therapy, (ii) administering entinostat-exemestane combination therapy, and (iii) determining the level of protein lysine acetylation during the course of said therapy, wherein the level of protein lysine acetylation is determined in a tissue sample obtained about 15 days after initiating therapy; or (d) treating breast cancer which displays resistance to prior aromatase inhibitor therapy, the method comprising administering to a patient a combination comprising entinostat and exemestane, wherein the patient did not demonstrate a complete response, a partial response or stable disease for greater than six months during prior treatment with an aromatase inhibitor, and wherein the method further comprises determining the level of protein lysine acetylation in a tissue sample obtained about 15 days after administration of the combination.