In summary, 3 years of diagnostic testing of TNNI3K in arrhythmia and cardiomyopathy patients yielded 2 missense variants (TNNI3K-p.Ile512Thr [1 family] and TNNI3K-p.His592Tyr [7 families]) in TNNI3K with cosegregation with DCM, SVT, and CCD. Here, TNNI3K is linked to familial dilated cardiomyopathy.