APP and Alzheimer disease: In addition, the contribution of circulating monocytes (defined by low expression of Ly6C) was examined through the creation of blood chimeras between APP/PS1 mice, a mouse model of AD, in which bone marrow was removed by the administration of the antineoplastic drug busulfan/cyclophosphamide, and Nr4a1-KO mice, in which circulating monocytes were absent.