FOS and theileriasis: Since attenuated vaccine lines used to fight tropical theileriosis are derived by long-term culture of virulent infected macrophages, and since many virulence traits can be restored by TGF-β2–stimulation of attenuated macrophages [5], loss of Theileria-infected macrophage virulence such as activation of the Activator Protein 1 (AP-1) transcription factor [6, 7] may also have an epigenetic element [8, 9].