ABCA4 and glycogen storage disease V: Patient 4 had been diagnosed previously with Stargardt disease, but genetic testing yielded no mutations in ABCA4. The results of this study are of clinical significance because they support the association of this retinopathy with McArdle disease and can reduce the likelihood of misdiagnosis; this is increasingly important because particular genetic causes of retinopathy (including ABCA4) are subject to a number of novel treatment trials.