SCFD1 and amyotrophic lateral sclerosis: Surprisingly, skeletal muscle was also found to be the relevant tissue for SCFD1. Although the weakening of skeletal muscle was thought to be the initial hallmark of ALS, whether ALS originates in peripheral tissues (dying-back phenomenon) (Dadon-Nachum et al., 2011), including skeletal muscle or motor neurons (dying-forward phenomenon) (Braak et al., 2013) has been fiercely debated.