Sets of genetic markers for specific tumor classes are described, as well as methods of identifying a biological sample based on these markers. Genes that are differentially expressed between different stages of colorectal tumors are identified. Genes that are differentially expressed between tumors that have metastasized to the regional lymph nodes and tumors that have not metastasized to the regional lymph nodes are identified. Changes in gene expression pattern between two samples or between a sample and a control may be used to determine tumor stage. Also described are diagnostic, prognostic, and therapeutic screening uses for these markers, as well as oligonucleotide arrays comprising these markers.