SPHK1 and Alzheimer disease: In addition, an alteration in Sphk1–S1P activity has been found to be dominant in some CNS regions compared with that in other regions, i.e., decreased S1P expression, reduced Sphk1 activity, and elevated S1P lyase levels were detected in the CA1 region of the hippocampus and gray and white matter of the temporal gyrus in patients with AD; both these regions regulate memory formation and spatial learning in the brain [56].