Initially, cases of HMSN4 not corresponding to CMT4 were described as the recessively inherited phytanic acid metabolic disorder Refsum disease (involving mutations of phytanoyl-CoA hydroxylase and PHYH ablation of alpha-oxidation and resulting in phytanic acid accumulation), which has childhood onset and systemic multiorgan involvement including ichthyosis, retinitis pigmentosa, and ataxia [17]. The gene discussed is PHYH; the disease is cerebellar ataxia.