SLC17A5 and free sialic acid storage disease: Mutant forms of sialins in persons suffering from ISSD show complete absence of H+/sialic co-transport activity, whereas the mutant forms of sialins found in Salla disease patients still exhibit 20–60% of normal H+/sialic co-transport (Morin et al., 2004; Wreden et al., 2005; Myall et al., 2007; Ruivo et al., 2008).