PSEN1 and Alzheimer disease: However, the GSK3β protein was expressed at significantly higher levels in the hippocampus of transgenic APP/PS1 mice than in the hippocampi of non-transgenic littermate mice, while the hippocampal level of GSK3β phosphorylated at Ser9 was significantly lower in the transgenic AD mouse model than in the hippocampi of non-transgenic littermate mice.