Likely pathogenic variants in SLC17A5 results in allelic disorders of free sialic acid metabolism including (1) infantile free sialic acid storage disease with severe global developmental delay, coarse facial features, hepatosplenomegaly, and cardiomegaly; (2) intermediate severe Salla disease with moderate to severe global developmental delay, hypotonia, and hypomyelination with or without coarse facial features, and (3) Salla disease with normal appearance, mild cognitive dysfunction, and spasticity.