Provided herein are methods for identifying a subject afflicted with chronic lymphocytic leukemia who is responsive to treatment with a chemotherapeutic agent by detecting the presence or absence of at least one APOE4 allele in the subject, the presence of an APOE4 allele identifying the subject as responsive to the treatment. Also provided are methods of treating a subject afflicted with chronic lymphocytic leukemia, including administering an estrogenic agent, an androgen withdrawal agent, an apoE4 peptide or mimetic thereof, and/or a chemotherapeutic agent in an amount effective to treat said chronic lymphocytic leukemia. Methods of determining a prognosis for a patient diagnosed with chronic lymphocytic leukemia are also provided. In addition, methods for stratifying a subject into a subgroup of a clinical trial and methods for identifying a patient in a clinical trial of a treatment for chronic lymphocytic leukemia are herein provided.