PHF6 and early-onset autosomal dominant Alzheimer disease: On the other hand, Ser285 and Ser289 near the C-terminus of the PHF6* and PHF6 sequences are known to be phosphorylation hotspots for casein kinase 1 (CK1), casein kinase 2 (CK2), and glycogen synthase kinase-3 (GSK-3) in the brains of Alzheimer’s disease patients.