Abstract
This paper presents the case of a 14-year-old boy with Danon disease exhibiting exclusive cardiac involvement, initially suspected as hypertrophic cardiomyopathy. Significant deterioration raised suspicion of an alternative etiology. A clinical exome revealed a previously unreported likely pathogenic variant, LAMP2:c.645_651del (p.LysAsnfs∗25), in hemizygous state, confirming the diagnosis of Danon disease.