Background: With the discovery of the very promising, germ cell malignancy (GCM) specific, liquid biomarker microRNA 371a-3p (miR371), the investigative trajectory has markedly accelerated. With its outstanding, previously-reported specificity and positive predictive value, miR371 likely will become a powerful tool for clinical decision-making. The primary objective of SWOG S1823/CCTG GCC1 [NCT03067181] is to define the operating characteristics of plasma miR371 expression at the time of clinical relapse for low/moderate risk non-seminoma GCM patients on active surveillance. Methods: S1823 is a prospective, observational, adult GCM translational trial which is actively accruing. Broad eligibility includes all newly diagnosed adult GCM patients. Patients are assigned to low (<25% risk of relapse), moderate (25-90% risk of relapse) or high (≥90% risk of harboring active GCM) risk groups. Pragmatic logistics include using Streck tubes and centralized processing for miR371 sample processing and analytics. Research samples were drawn at the time of routine blood draws minimizing patient burden. Source documents are submitted along with case report forms. This data-gathering model gives an important data quality-control check. S1823 began in July 2020 and has since accelerated its accrual to consistently predicted new enrollments of 20-30 cases/month. As of January 2023, the study enrolled 389 low-risk, 114 moderate-risk and 146 high-risk patients (657 total). Interim analysis is planned at the time 40 non-seminoma GCM patients have relapsed and is anticipated in late 2023 to early 2024. 404 centers have opened S1823. 3 Canadian centers have enrolled >10 patients (range 11-51). 17 USA institutions have enrolled >10 patients (range 11-112). In North America, 9 of the 12-storied GCM clinical research programs have had robust participation. The leading accruing organization was the Kaiser Permanente system where 112 patients have been enrolled. In Canada, all population centers contributed proportionally. In the USA, the dominant enrollment comes from the west coast and mid-west. Proportional enrollments have been seen in Hispanic and Asian populations. The entire study provides rich opportunities for a variety of secondary use and patterns of care projects that will begin to roll out over the next year. Clinical trial information: NCT03067181.