Severe alleles in PEX genes, including PEX2C247R and PEX16R176*, lead to severe clinical presentations; however, in recent years, mild clinical presentations due to PEX mutations have expanded from phenotypes referred to as infantile Refsum disease that includes atypical ataxia in the PEX16F332del variant, and Heimler syndrome associated with variants of PEX1 and PEX6 (Rosewich et al., 2005; Waterham and Ebberink, 2012; Bacino et al., 2015a,b). The gene discussed is PEX1; the disease is Refsum disease.