However, given that ET-1 (∼40 seconds) and big ET-1 (∼23 minutes) have short half-lives,33,37 intermittent elevations in circulating ET-1 and big ET-1 levels produced by the primary tumor or residual cancer cells might be masked and/or the time of day collection of blood may also contribute to changes in ET-1 levels.38 This evidence concerns the gene EDN1 and cancer.