Genetic markers in the CSF2RB gene associated with adverse hematological response to drug therapy are disclosed. Compositions and methods for detecting and using these CSF2RB markers in a variety of clinical applications are disclosed. Such applications include methods for testing an individual for susceptibility for an adverse hematological response, methods of selecting the appropriate drug therapy for patients based on the presence or absence of a CSF2RB marker, and products comprising a drug with hematological toxicity that are approved for treating patients lacking a genetic marker.