Here, we report the case of two siblings diagnosed with severe, early onset pediatric DCM associated with compound heterozygous variants in TNNC1. As a result of our attempts to understand the molecular basis of these pediatric DCM cases, we provide in vivo experimental evidence confirming that TNNC1 is a crucial gene for heart function, and further provide in vitro experimental evidence that altered contractile mechanics – unexpectedly – cannot always explain the pathogenicity of TNNC1 variants. The gene discussed is TNNC1; the disease is familial dilated cardiomyopathy.