The rarest subtypes of mucopolysaccharidosis type III, namely subtype C [OMIM:252930] and D [OMIM:252940], are both hyper-rare AR LSDs with an unknown prevalence, caused by genetic mutations of two different genes: the HGSNAT gene (8p11.2-p11.1) and the GNS gene (12q14.3), respectively. The gene discussed is GNS; the disease is mucopolysaccharidosis type 3.