Provided herein are methods for identifying a cancer patient at risk for resistance to an HDAC inhibitor therapy, comprising obtaining a tumor sample from the cancer patient; detecting the presence of Testis-specific Y-encoded-like protein 5 (TSPYL5) expression in the sample; quantifying a level of the TSPYL5 expression in the sample, wherein a high level of the TSPYL5 expression, relative to a defined expression threshold of the TSPYL5, correlates with resistance to the HDAC inhibitor therapy; and applying the correlation to identify the cancer patient at risk for resistance to the HDAC inhibitor therapy. Also provided is a method for identifying a cancer patient with an increased likelihood of a positive clinical response to an HDAC inhibitor therapy comprising obtaining a tumor sample from the cancer patient; detecting the presence of Testis-specific Y-encoded-like protein 5 (TSPYL5) expression in said sample; quantifying a level of said TSPYL5 expression in said sample, wherein a low level of the TSPYL5 expression, relative to a defined expression threshold of the TSPYL5, identifies said cancer patient with an increased likelihood of a positive clinical response to said HDAC inhibitor therapy. Related methods and compositions are also provided.