SLC17A5 and free sialic acid storage disease: Defective sialin leads to the accumulation of free sialic acid in lysosomes and results in a clinical spectrum ranging from the severe infantile free sialic acid storage disease with global developmental delay, coarse facial features, hepatosplenomegaly, and early death, to the milder Salla disease characterized by normal appearance, mild cognitive dysfunction, spasticity, and motor disability.