Vylet’al P, Kidd K, Ainsworth HC, et al. - Given ADTKD-MUC1 (autosomal dominant tubulointerstitial kidney disease due to MUC1 mutations) patients carry one allele producing normal mucin-1 (MUC1) and one allele producing mutant MUC1, which remains intracellular, researchers undertook this cross-sectional analysis to investigate whether ADTKD-MUC1 patients, who carry only 1 secretory-competent wild-type MUC1 allele, demonstrate reduced plasma mucin-1 (MUC1) concentrations. For this purpose, they repurposed the serum CA15-3 assay (a widely available commercial assay that measures blood levels of MUC1 using the DF3 antibody) employed to determine MUC1 in breast cancer to measure plasma MUC1 concentrations in ADTKD-MUC1. Experts assessed CA15-3 concentrations in a reference sample of 6,850 people, in 85 persons with ADTKD-MUC1, and in a control sample involving 135 persons with ADTKD-UMOD and 114 healthy people. Findings revealed the presence of approximately 40% lower plasma CA15-3 levels in ADTKD-MUC1 patients when compared with levels in healthy people, though a significant overlap between groups was observed. There is a requirement for further investigations to ascertain whether plasma CA15-3 levels would be beneficial in diagnosis, prognosis, or evaluating response to new therapies in this disorder.