Inclusion Criteria:  Must have given informed consent indicating that they understand the purpose of and procedures required for the study and are willing to participate in the study. Age ≥ 18 years old at time of signing the informed consent form. Ability to comply with the study protocol, in the investigator's judgment.  Must have Stage II/Stage III rectal cancer or Stage II (high risk)/Stage III colon cancer per AJCC 2017 that has been surgically totally resected (R0 confirmed by pathology report). Stage II (high risk) colon cancer is defined as (any of):  T4 Grade ≥ 3 Clinical presentation with bowel obstruction or perforation Histological signs of vascular, lymphatic or perineural invasion < 12 nodes examined Adequate tumor material in formalin-fixed paraffin embedded (FFPE) blocks or as sectioned tissue (only upon approval by sponsor) must be available, preferably from resection. The specimen should be submitted along with an associated pathology report. Multiple samples may be provided as available, but priority should be given to tissue with the highest tumor content and lowest necrotic area. Intention to receive a standard of care adjuvant chemotherapy (AdCTx) within 8 weeks post-surgery, and be scheduled for at least 3 months of treatment according to the treating physician or investigator. Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1. Adequate end-organ function.  Exclusion Criteria:  Induction of neoadjuvant systemic therapy prior to resection of CRC. Prior systemic investigational therapy.  Positive serology for hepatitis B (unless immune due to vaccination or resolved natural infection or unless passive immunization due to immunoglobulin therapy):  Positive test for antibodies to hepatitis B core antigens (anti HBc) and Negative test for antibodies to hepatitis B surface antigens (anti HBs). Active hepatitis C virus (HCV) infection; participants who have completed curative antiviral treatment with HCV viral load below the limit of quantification by polymerase chain reaction (PCR) are allowed. Participant has a history of human immunodeficiency virus (HIV) antibody positivity, or tests positive for HIV at screening. Residual tumor classification following surgery other than R0 (microscopic margin-negative resection). Diagnosis with disease other than Stage II/Stage III rectal cancer or Stage II (high risk)/Stage III colon cancer per AJCC 2017 following surgery. Participant has not started standard of care AdCTx within 8 weeks post-surgery. Participant has received less than 3 months of AdCTx treatment. Inadequate tumor material (either quality and quantity) to support circulating tumor DNA (ctDNA) analysis.