To the Editor: Blanco-Domínguez et al. (May 27, 2021, issue)1 describe a potential microRNA blood-based assay for the detection of myocarditis that fills an important diagnostic gap. Unfortunately, the origins of the amplified RNA segments are unclear. The mouse mmu-miR-721 has generally been refuted as a bona fide microRNA.2,3 The proposed human homologue, which the authors called hsa-miR-Chr8:96, is not in a homologous chromosome region and has a structure that would not support microRNA processing (Fig. S1 in Supplementary Appendix 1, available with the full text of this letter at NEJM.org).2,4 There is essentially no precedent for microRNAs that swap . . .