Intracellular filamentous inclusions composed of amyloid-like proteins are common neuropathological features of many neurodegenerative disorders. Although the extent of the abnormal protein pathologies is closely correlated with the disease progression, little attention has been given to the molecular mechanisms to explain how these pathological proteins spread. We developed a novel method for introducing amyloid seeds into cells, and presented experimental evidence of seed-dependent polymerization, leading to the formation of filamentous protein deposits and cell death. Overexpression of alpha-synuclein itself does not generate abnormal inclusions, but if fibril seeds are introduced, abundant alpha-synuclein inclusions positive for PSer129 and ubiquitin are developed, and the cells undergo cell death. This was also clearly demonstrated in cells expressing different tau isoforms by introducing the corresponding tau fibril seeds. These results obtained from biochemical analyses of abnormal proteins in patients strongly suggest that amyloid-like proteins, including tau, alpha-synuclein and TDP-43, propagate from cell to cell and this propagation is the cause of disease progression, analogously to metastasis of cancer cells to multiple different tissues in cancer progression. From this point of view, I have proposed as a hypothesis that neurodegenerative diseases with amyloid-like proteins can be regarded as "protein cancers".