**2. Materials and Methods**

#### *2.1. Patient Enrollment and Phenotyping*

In the present study, we used data generated in the Care4DCM multi-omics project [8,14]. The diagnosis of DCM was established by impaired heart systolic function, after ruling out coronary artery disease through coronary angiography. Notably, aside from DCM patients with advanced heart failure, patients in early stages of the disease, i.e., left ventricular ejection function > 45% and < 55%, were also included in the present study. Patients with concomitant valvular heart disease, myocarditis, and inflammatory DCM were excluded from the study after being screened with echocardiography and cardiac magnetic resonance imaging (CMR), and after the myocardial biopsy of the patients were examined histopathologically. Other exclusion criteria included previous cardiotoxic chemotherapy, alcoholism, illicit substance abuse, and untreated arterial hypertension. For control subjects, after written informed consent, we collected 31 specimens of left ventricle biopsy from asymptomatic patients after heart transplantation at least 6 months ago. All patient recruitment was authorized by the ethics committee. An independent replication cohort comprised 18 explanted DCM hearts and 8 healthy hearts from traffic accident victims. The following library preparation and analytic

procedures were identical in both the screening and replication cohorts. Further information regarding patient enrollment and phenotyping can be found in our previously published study [8].
