TIE1 and viral infectious disease: Moreover, monitoring of defined cells in brain slices can be achieved by acquiring transgenic animals expressing live-cell reporter genes in a cell-type specific manner such as CX3CR1 (for microglia), GFAP (for astrocyte-specificity) or tie1 (for endothelial cells) and by ectopic gene expression through ectopic transfection or viral infection [24].