1. A method of identifying the origin of a neoplasm of unknown primary comprising: obtaining a sample of a neoplasm; obtaining morphological data of the neoplasm; obtaining cytokeratin immunohistochemical data of the neoplasm; developing microarray-based gene expression classifiers for the neoplasm; and correlating the morphological data, immunohistochemical data, and microarray-based gene expression to the tissue origin of the neoplasm.