INS and neoplasm: β-cell hyperplasia was also induced in Pdx1-tTA; tet-o-PyMT-IRES-Luc bitransgenic mice, and the insulin-expressing β-cell hyperplasia does not further progress to form frank neoplasms even after 1 year of oncogene activation and in Ink4a/Arf-null and Arf-null backgrounds.