APP and brain neoplasm: MC65 cells are a human CNS nerve cell line that contains the C-99 fragment of the amyloid precursor protein under the control of a tetracycline (tet)-sensitive promoter.12 The parent cell line is SK-N-MC from a human brain tumor, and it has an electrically excitable membrane typical of neurons.13 When tet is withdrawn, cells express C-99 that is converted to Aβ by γ-secretase and the cells die within 4 days (Figure 1a,b).