In the context of a phenotype consistent with recessive <i>MYL2</i>-associated disease, these findings support a likely pathogenic interpretation.<h4>Conclusions</h4>This case expands the allelic and phenotypic spectrum of recessive <i>MYL2</i>-associated cardiomyopathy and highlights the value of early genomic testing in infants with unexplained hypotonia and rapidly progressive cardiac dysfunction. Here, MYL2 is linked to cardiomyopathy.