POPDC1 and cardiac rhythm disease: POPDC proteins are a family of membrane proteins that are found in cardiac and skeletal muscle cells, among other tissues, and are encoded by the three genes POPDC1, POPDC2, and POPDC3. They have high affinities to cAMP, and mutations in the POPDC1 gene have been implicated in limb-girdle muscular dystrophy and cardiac arrhythmia (18, 19).