The ARC Registry is a large-scale, multi-site, prospective, observational registry. All patients with a solid tumor malignancy who receive Ataraxis testing as part of routine clinical care are eligible for enrollment, which will link Ataraxis test results with short- and long-term patient outcomes to evaluate the real-world performance and clinical utility of Ataraxis tests. The ARC Registry utilizes an adaptable design, where additional Sponsor-led analyses and investigator-initiated ancillary studies can be added after the initial study is opened.