Six HCM patients had known phenocopies on the basis of a genetic variant, out of which we confirmed all six: four patients with heritable transthyretin amyloidosis carried LP/P variants in TTR, a patient with glycogen storage disease IV was a carrier of a P variant and a rare VUS in GBE1, and a patient with Danon disease was a carrier of a P variant in LAMP2. The gene discussed is TTR; the disease is Danon disease.