GNE myopathy, a type of distal myopathy with rimmed vacuoles, is a hereditary muscle disease characterized by particularly distal muscle weakness and is an ultrarare disease with no effective treatment [1]. It is estimated that there are about 400 patients in Japan, and it is counted as one of the designated intractable diseases. GNE myopathy is caused by mutations in the GNE gene, which encodes the UDP-N-acetylglucosamine (UDP-GlcNAc) 2-epimerase/N-acetylmannosamine kinase (GNE) [2], the first enzyme in the synthesis of N-acetylneuraminic acid (Neu5Ac) from UDP-GlcNAc, the most abundant sialic acid species in vivo (Fig. 127.1). This mutation has been identified to cause a decrease in Neu5Ac level. Subsequently, clinical trials of Neu5Ac supplementation therapy led by physicians have been conducted since 2010, and its effectiveness and safety have been confirmed [3–5]. Neu5Ac is a mixture of cyclic and open-chain forms in solution, with most existing in a cyclic structure. The open-chain compound of Neu5Ac is called aceneuramic acid, and acenobel sustained-release formulation is the pharmacological name for aceneuramic acid.