Fig4, a component of the PIKfyve complex required for generating the late endosomal lipid PtdIns3,5P2, is mutated in individuals with CMT4J (Chow et al., 2007), which shares morphological similarities to CMT1C (Edgar et al., 2020). The gene discussed is RNMT; the disease is Charcot-Marie-Tooth disease type 4J.