The most relevant rare diseases associated to RFUO encompass systemic autoinflammatory diseases (SAIDs), for which genetic tests are usually required (including mutations in the following genes: MEFV, TNFRSF1A, MVK, NLRP3, NOD2, etc.), and lysosomal storage disorders (LSDs) (including FD, Gaucher disease, Pompe disease, mucopolysaccharidoses, etc.), for which specific enzyme assays and genetic tests are required. This evidence concerns the gene TNFRSF1A and mucopolysaccharidosis.