The present disclosure provides combination therapy comprising a BET inhibitor and a protein phosphatase 2A (PP2A) activator, a B-cell lymphoma-2 (Bcl-2) inhibitor, a B-cell lymphoma-extra large (Bcl-xl) inhibitor, a casein kinase 2 (CK2) inhibitor, and/or a mediator complex subunit 1 (MED1) for cancer. The combination therapy is expected to be synergistic in treating the cancer, compared to the monotherapy. Methods for identifying a subject having a cancer that is resistant to or at risk of developing resistance to bromodomain and extra terminal (BET) inhibitor therapy are also provided.